Forced to restart, seemingly at random. why?

Hello All,

I wasn't running anything heavy as far as I know, just streaming a radio station on itunes, and I was using Chrome when i got the 'grey screen of death': "You Must restart your computer now by pressing power button....."

This was the second time that this happened, but the first happened when i powered off a usb device that was acting as a 'drive' and i didn't eject it first.

What next? I copied the details of the report that was given if that helps.

Thanks in advance

imac 2011, 10.7.3

 

Okay, thanks for your help.

I did get a report, though. After I rebooted I got the option to send info to Apple because it didn't restart properly.

Anyways, cheers