Windows 8 getting rebooting unexpectedly

I am using an assembled PC. My OS is Windows 8 Pro. Processor is Intel Pentium Dual Core. Mother board is also from Intel. 3 days before my PC get shut downed without any reason. It was sudden, the CPU just got off without any warning or showing anything, and it automatically booted after a second. It happened today too for 2 times. Why is it so ?

The .DMP file is great to see what caused the error if you have a driver or software issue. You can also check the event log maybe something was logged but sometimes because of the error it may not show anything. Also check the big two hardware causes. Run a RAM checker. Make sure your HD is not going bad or has bad sectors.

From the little information provided, I wonder if this isn't more likely to be a processor issue.

I would recommend installing a utility to monitor processor temperatures and run a test to stress out the processing and memory. Watch out for excessive heat

As this is a home-built PC, have you moved it recently? It might be worth unplugging and reseating both the RAM and the processor. Also, have you checked the fans & air intakes/exits? Are they the right way round? (You'd be surprised how many people make that mistake) and are they pushing air through the PC properly?

Yes, also, it could be your BIOS may be set up to shut down/boot the machine at specific times. Reboot, go to the BIOS "Power options" and check. Also some software can run a command to shut off the system. Plus be sure you installed the fans correctly. There are free apps to check if the processors/cores are overheating, one is called Core Temp.