Yep same thing happens to me, run low on ram and the undo history clears out. It would also clear the history on me when it auto saved, that stopped happening in 2012 but there are other times it stops responding...
There are times you might be in a sub-menu or a name box where ctrl-z won't work but the undo history hasn't been cleared. This happens to me when animating, I use undo/redo to flip between two changes and there are times it just doesn't respond. In those cases you can try the buttons in the upper left by the giant file menu button. Clicking the little drop down arrow next to the undo arrow, will show you if the history is there or if its been cleared.
In older versions of max you can try Main Menu> Edit > Undo. Typically it will say "undo bla bla bla" if its gray'ed out then its probably been cleared and you're on your way to a crash. so you should restart.
Last edited by Mark Dygert; 08-11-2011 at 07:23 AM..