Laptop with Ubuntu 10.10 shuts down after a period of inactivity

I recently upgraded Ubunto 10.04 to version 10.10 on my laptop. I have noticed that if there is a long-ish period of inactivity the laptop shuts down and I have to re-enter my user name and password rather than just my password as is the case with my desktop.

I was using the Update Manager just now and at some point the laptop shut down. When I restarted it it gave an error message about a package being broken but apart from that it appeared to be OK so I started Update Manager again thinking that it might carry on from where the computer shut down but it wanted to do a complete upgrade. I stopped it from doing that and rebooted but now it gets to the point where I have to enter my user name and then hangs without letting me enter my user name.

Short of re-installing it is there anything that I can do to extricate myself from this mess? I have tried running it is recovery mode but it only gets so far and then hangs. I have noticed that it now has two versions of the kernel but I can't boot the machine with either of them. As well as recovering from this, what can I do to stop the laptop from rebooting after a period of inactivity rather than just switching the screen off. It has never done this before with Ubunti 9.* or 10.04 - only with 10.10