This happened to me in Ubuntu 11.10 (Oneiric Ocelot) when I was playing with some Compiz settings. To have your shortcuts back, go to the CompizConfig Settings Manager, to do that bring up a terminal window with the shortcut ctrl+alt+t the type ccsm:
imame@meta:~$ ccsm
If you are using Unity open Desktop > Ubuntu Unity Plugin but if you are using Gnome shell choose General > Gnome Compatibily – anyways, choose whichever is already checked, one the next window check that the shortcuts are enabled and configured correctly
