Thank you, Brianzion and Bill2, for your almost immediate replies!
Unfortunately, Brian, I can't use your solution since I'm still stuck at the login screen, which is why I had to post this question.
Bill, there is a drop-down list (although it's at the upper-left corner of the login screen, not the bottom-left), but it only has one entry and it's "Hebrew." That's how I figured out what's wrong, although I still don't understand how the US layout got lost.
So, back to the original question.