Maybe try this
Gemma,
On a couple of Win 7 machines, I have had a similar issue which appears to be due to the USB power settings. As I have never been able to ascertain the exact point of seizure, I have never been absolutely certain that this was really the cause or the following the fix but, assuming your mouse is connected via USB, it does seem to have cured the problem and is anyway surely a fairly harmless adjustment.
From Device Manager, click on the cross next to USB Controllers (at the bottom of the list) and from the list that will appear, select each generic USB hub or USB Root hub, open the Power Management tab and un tick the box' Allow the computer to turn off this device...'
Ideally, you need only do this to the actual item that is connecting the mouse but you will have to determine which one that is.
The logic of this, (which information I originally found online), seems to be that after a certain period of inactivity, the machine ignores the mouse and then will not 'wake up'. This seems like a ridiculous simple failure and error to have been incorporated in such software.
One of many circumstances where Windoesnot might be the more appropriate name...
Do try and report back maybe.
Good luck
jg