My comment was greyed out but not saved... meh.

my guesses:
1) keyboard layout was changed from standard US to DE, FR, RU or whatever else, so a Ctrl+Z is no longer a Ctrl+Z;
2) a newly-installed app has hijacked shortcuts (Intel video drivers and some Polish national diacritics, anyone?)
3) a retrieve has been made with Excel add-in for Essbase (granted, this is rather exotic, but does happen... and hurts when it does).