OS X 10.10.,3, G2 [749].
Same exact symptoms as earlier occurrence in 2013. Routine editing of a font (sans) and after an undetermined time Cmd-Z quit working. Quit G2 and restart, it works once again.
This is an old bug that comes and goes. You get used to it after a while.
This is most likely caused by a unfortunate combination of a script or plugin that breaks undo. I can’t reproduce it. So if you can narrow it down what tool you used before it breaks, that would help a lot.