Are you on a Mac? In that case, it’s Cmd+Z.
For those who are interested in how I restored the file - there’s a Git repository built into every project, and you can run Git commands from the command line. I did a
git log to look at the last few commits, and then
git diff <hash> to look at the changes in each one. In this case, the second-last commit had the tictactoe.js with code in it, instead of the empty version. So I ran
git checkout <hash> util/tictactoe.js to restore only that file to the earlier version, and then
refresh to tell the editor to reload the file from disk.