If you right click on My Computer and take Manage, then Event Viewer and System, see if the system logged a failure. What comes to mind is an over temperature shut down. Frequently after 3 years enough dust accumulates in the fans, especially the fan mounted on the processor chip heat sink that it stops the air flow, the system senses heat and shuts down to protect itself. There should be some kind of log entry to say why it shut down unless it's a power problem, loose wire or something like that. Assuming it was a logical shut down for self protection, there should be a log entry.