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. http://www.newtechnology.co.in/asus-m3a78-eh-amd-780g-motherboard/
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.
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).
I would keep track on the temps of your cpu,noth bridge and vga. If one is over heating, that could be the issue. With a normal bsod you can still restart your pc.
Just because you have plenty of air flow, doesn't mean that you have the proper amount of thermal past on your components.
When you get a bsod, how long do you have to wait before your pc will start up again?
When you restart your pc how long will it run before bsod? will it work indefinitely until you play a game again?
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My cpu is not starting it doesn't get power, i removed everything but it still is not starting. I am taking to expert who can resolve this but i need help in one thing that what i do because this happens 3-4 times with me. I blowed up two psu while playing world in conflict which is of 450 watts and now 700w. So what do you suggest that i purchase to make it work correctly. If you need any information please reply or mail me at firstname.lastname@example.org. Sorry for my bad english. Thanks anyway.