How to restore Chrome's list of recently closed tabs

Turn off Chrome while you do this, to avoid any possible conflict over who is editing or changing files.

Find Current Tabs in Time Machine (or whatever backup software you're using) from a time before Chrome lost your tabs.

You'll find it here on a Mac:

~/Library/Application\ Support/Google/Chrome/Default/Current\ Tabs\

or if you use Chrome Canary:

~/Library/Application\ Support/Google/Chrome\ Canary/Default/Current\ Tabs

Rename the existing Current Tabs to something else, and restore the one from your backup.

Start Chrome.

You should now see the recently closed tabs under History → Recently Closed.

(If you do not have a backup, or just don't have one handy, you can also try using Last Tabs. It seems like it ought to work, but I have not tested it.)

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UPDATE: Since this is likely to happen again (and again and again and...), it's a good idea to take pre-emptive measures to be prepared. The best pre-emptive measure that I've found to make session recovery easier when Chrome drops the ball is to install the Session Buddy Chrome plugin. It does a far better job than the built-in session management, and also better than any other plugin I've tried. (It's also the highest rated, at the moment I'm writing this.)