I failed to catch your item that the system won't restart.
Based on that, my first suspect is the video card or RAM has failed.
However, we need MORE INFORMATION.
When you switch the system on, what happens? Does anything show on the screen? Are there any beeps? Is the PSU fan spinning? Does the CPU fan spin?
edit to add:
Try reseating your video card (with the PSU's AC power cord removed!!!). Then reboot.
If it still doesn't work then...
I also just looked at the specs for the mobo, and it says it has integrated HD3200 video.
So, remove the nVidia graphics card, and connect your display to the motherboard video connector, then reboot. If it works, then it's quite possible your video card has failed.
MY OLD POST just so if you get system running you can re-examine
You didn't provide much information about the actual failure, but you might try 'bluescreenview'. If your system is restarting automatically, you have to change the settings so it instead creates a minidump file (e.g. shows the BSOD (Blue Screen Of Death))
Bluescreenview helps you determine if software drivers are contributing to a problem and if so which ones are involved. Drivers are often reported as contributing to BSOD during high-stress video. Sometimes going back a driver level or two, or installing the latest video drivers helps.
"BlueScreenView works with Windows XP, Windows Server 2003, Windows Server 2008, Windows Vista, and Windows 7, as long as Windows is configured to save minidump files during BSOD crashes."
edit to add:
If you have your OS and programs set to auto-update, it's also possible an software update has caused the problem. you might look at your history log and see if any recent updates were made, maybe even roll-back to the last known good software state (before rolling back to a prior restore point however, search the internet for Restore Point and understand the things that happen when you do that).