I saved over my manuscript by accident

If the file is still open

Do not close it. Undo usually survives a save within the same session, so press Ctrl+Z (⌘+Z on a Mac) until the old text is back, then save it under a new name. Once the window closes, that route is gone.

If you pasted a different file's contents over it, the same applies. And stop saving to that file name while you work out the rest; every save pushes the good version further back.

Earlier versions the system kept

• OneDrive or SharePoint. In Word, File › Info › Version History (on a Mac, Browse Version History, which opens from the file name at the top of the window). On the OneDrive website, the file's context menu offers Version history as well. Dropbox and Google Drive keep versions of synced files too, reached the same way from their websites.
• Windows File History. Right-click the file, open Properties, and look at the Previous Versions tab. It lists something only if File History or restore points were already turned on.
• Time Machine on a Mac. Open the folder in Finder and open Time Machine from the menu bar or the Applications folder. Step back to a date before the accident, select the file, and choose Restore.
• Apps that use macOS versions. Pages and TextEdit keep their own history: File › Revert To › Browse All Versions. Word for Mac relies on OneDrive's history instead.

Then think about who else holds a copy. A manuscript sent to an agent, an editor, or a beta reader is sitting in your sent mail, and even an older draft from there beats rewriting.

In Tabularia

There is no Save or Save As, so this exact accident has no counterpart: nothing asks which file to write to. Importing a file makes a new manuscript rather than landing on top of the one you have. The nearer equivalent is replacing text inside the book, a chapter pasted over or cut by mistake.

For that, undo reaches further than a session. Undo/Redo History lists the edits by day, though it never leaves the computer it was recorded on and holds only up to the number of steps the project keeps, 250 unless you change it. A Snapshot goes further back, and restoring one first takes a Snapshot of where you are, so the restore can be walked back. One thing a restore does discard: unresolved Review Changes and any Review Comment added or changed since, which the dialog counts for you before you confirm.

Snapshots are taken on purpose, not on a timer, so they cover only what you remembered to mark. And on the Free plan they live beside the manuscript in one browser. For a copy outside it, the guide's advice is an export at milestones.