Sometimes, with my laptop, Kubuntu 9.10 will not resume, it merely gets stuck at a black screen when I reopen the screen.
The workaround that I have found for this is simple. Press CTRL+ALT+F2 to switch to a virtual terminal and login as your usual user.
Type:
startx
and press enter. After about 5 seconds press CTRL+C to abort this process. Then press CTRL+F7 to return to your previous session.