My computer randomly shutdown while playing a game.
It started beeping. 3 long beeps, 1 per second, then repeated after 5 seconds.
I just installed a new GFX card (750 Ti), and reseated the CPU heat sink.
How do I diagnose the problem?
3 long beeps is normally a memory issue but you don't mention what BIOS you have.
Since you've installed hardware, ensure the RAM is seated well.
Try with just 1 stick of RAM, and then swap it to see when/if the beep continues/stops. Then maybe use MEMTEST...
Read more about beep codes http://www.computerhope.com/beep.htm
However, I would first ensure everything is seated. It could be the new GPU isn't seated correctly...
Based upon now knowing your BIOS in the comments, here is the same question on a forum which also suggests it's faulty RAM https://communities.intel.com/thread/4817