Lubuntu 13.04 won't shutdown, keeps re-starting

Lubuntu 13.04 won't shutdown, keeps re-starting

I wanted to show my kid Minecraft running in Chrome on my Lenovo A55 PC
with Lubuntu 13.04. This unfortunately hung up the pc, in the end forcing
me to shutdown the PC by pressing the power button for several seconds.
After this, Lubuntu won't shutdown when I select this in the (logout) menu
- it just keeps restarting. Needless to say, it worked just fine until I
did the forced shutdown.
I did try "sudo update-grup", didn't help. The PC has dualboot, with
Windows XP, however I rarely use it.
I'd be grateful for any help - this problem is soooo annoying!