The Basics, Since You're Asking
Undo in Word is Ctrl+Z on Windows or Command+Z on Mac. You press it once to reverse the last action, twice to reverse the one before that, and so on. It tracks back through your recent activity until it hits something you saved, which acts as a hard stop for the undo stack. I used to think it was more complicated than that until I ran into a document where I deleted an entire section and then realized halfway through trying to redo my changes that undo only tracked changes back to the last save point, not backward through the session continuously. That meant if I had opened an existing file, made ten edits, and hit Ctrl+Z repeatedly, it would undo those ten edits and then stop rather than going further back into the document's earlier version. The workaround I ended up using was keeping a second copy open in a separate window so I could always cross-reference what the document looked like before my session started.
How Do I Undo In Word When It Stops Working
This happens more often than people expect, and it's not a bug, exactly. Word tracks individual actions as separate undo units, but certain operations consume multiple steps at once or replace the entire undo history. Inserting a table, for example, can clear everything you did before it from the undo stack. Pasting from certain external sources, like Excel sheets or web pages with heavy formatting, sometimes does the same thing. Macros are another common culprit, and embedded OLE objects will frequently wipe the stack clean. If you ever find yourself clicking Ctrl+Z with nothing happening, check whether you just performed one of those bulk operations. There's no way to retrieve what got wiped from the undo stack. The only option at that point is to use File > Info > Version History if you have AutoSave turned on through OneDrive or SharePoint, or open the previous version manually.
Keyboard Shortcuts Beyond the Obvious Ones
Ctrl+Z and Command+Z are the ones everyone knows, but there are two that matter just as much. Ctrl+Y and Command+Y redo the last undone action, which is useful when you go one step too far. Ctrl+G does nothing related to undo in Word despite what you might expect from Excel. The Edit menu in older versions of Word also had an Undo option with a dropdown that showed a list of your recent actions, each labeled clearly enough to let you click directly on the step you wanted to revert to rather than hammering Ctrl+Z repeatedly. Newer ribbon versions buried this, but you can restore it by right-clicking the Quick Access Toolbar and adding Undo from the commands list. It takes about thirty seconds to set up and saves time you'd otherwise waste clicking repeatedly.
Get the Full Details

What Word Considers an Action
This is where most people get tripped up. Word doesn't treat every keystroke as a separate undo event. It groups certain things together, which means pressing backspace three times to delete three characters counts as a single undo step. Typing out a full paragraph and hitting enter doesn't reset the undo counter until you do something else, like bold text or insert a page break. I learned this the hard way when I was rewriting a report and accidentally deleted half of a section, then spent five minutes pressing Ctrl+Z over and over expecting each press to bring back one line. It didn't work that way. Each press undid a grouped action, not an individual character. After that, I started paying attention to how Word handled grouping and adjusted my workflow to save more frequently so I wouldn't lose as much ground if the undo stack ever cleared unexpectedly.
AutoRecover and What It Actually Does
AutoRecover is not the same as version history. It saves temporary recovery files at intervals you set under File > Options > Save, usually every ten minutes by default. If Word crashes or your computer loses power, AutoRecover can restore the file to its state at the last recovery point. That might be ten minutes of work lost, or it might be nothing if you saved manually since your last auto-save. The limitation here is that AutoRecover files are just temporary snapshots. They don't track individual edits the way undo does, and they don't persist beyond the current session unless you explicitly save. I've had situations where I lost an hour of work because AutoRecover was disabled on a shared network drive, and the file was configured to only save to a temporary location instead of the actual destination path. Word crashed, and the only thing AutoRecover had was the document as it looked twenty minutes earlier. Turn on AutoSave if you're working from OneDrive or SharePoint. The incremental version history there keeps snapshots every few minutes, and you can go back to any of them through File > Info > Version History. It's more reliable than AutoRecover for anything longer than a few days of work.
Why Your Undo Stack Might Be Empty
Sometimes Ctrl+Z does absolutely nothing, and there's no obvious reason. A few causes that actually come up in practice: you opened the file from a network location where Word runs in reduced functionality mode, which disables some undo tracking; the document has tracked changes turned on, which changes how undo interacts with edits; or you're working in compatibility mode with an older .doc file format, and the older engine doesn't maintain the same undo depth as the modern .docx format. If you convert the file to .docx format and the undo starts working normally again, that was the issue. The compatibility mode workaround is to save as a new .docx file, close the old one, and work from the new version going forward. Conversion is usually immediate for documents without heavy legacy formatting, and it restores full undo capability along with a few other features that get stripped in compatibility mode.

Practical Workarounds for Common Failures
When undo fails you, the most reliable fallback is the Close Without Saving dialog that appears if you try to close a document with unsaved changes. Word asks whether you want to save, discard, or cancel. If you pick discard, you're back to the last saved version, which might be enough if you hadn't done much since your last save. If you picked save by accident, you can reopen File > Info > Version History to pull up the previous snapshot and manually copy whatever you need into the current document. Another thing people don't realize: the Undo dropdown in the Quick Access Toolbar, once you add it, shows the actual list of actions in order. You can click any entry in that list to jump directly to the state before that action occurred, rather than stepping through them one by one. This cuts down the time needed to reverse a series of changes from however many clicks it would take otherwise to just one click. I keep AutoSave on for every document I work on now, and I save manually before making any major structural changes like moving sections around or inserting tables. Between those two habits and the Quick Access Toolbar shortcut, I haven't lost meaningful work to a failed undo in about two years. That's not because undo got better, it's because I stopped relying on it as the only safety net.