Computer just completely froze, how do I find out what went wrong?

[Lysander]

As title, first time I've had this in W7, complete lockdown. Music stopped, cursor wouldn't move, couldn't do a thing. Had to restart. How do I find out the error?

 

[fornowagain]

Check the event viewer and look for critical events, right click the Computer Icon and open Manage.

 

[SiriusB]

Critical, Error and Warning may all point to a problem. Ignore Kernel-Power entries as that's when you probably had to hold the power button down to switch it off! The error you want will be close by to this entry.

Also check the Application log, not just System.