First, be sure the power cable is plugged in tight. All of them. Then, the next time you get the screen to come up, start windows in safe mode. Run an Anti-virus scan, remove any un-nessasary programs (like some cheap crap you might have downloaded from the net), then open up command prompt. type in "CHKDSK /f" without the quotes, press enter. Press "Y" when prompted to schedual the operation. Re-start your computer, and let the check run. If the computer keeps doing this or it won't restart again, take it in to a CPU repair shop. It costs money, but it may be the only way to fix it.
Also, before you do any of this, how old is your computer?
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