Computer does not boot..

Last week while playing at my PC,it suddenly hanged up. I tried to turned it off by pressing the button on the CPU but even for 30 seconds I held it, it didn't turn off. So I decided to directly turn if off by the AVR. And when I tried to turn it on the following day, the CPU did not boot but the lights were on and the fans turning properly but it did not show any signs of power on the big button in front of the CPU. No red and green lights. So the next day, I decided to put a thermal paste in the processor for I thought this was the problem. And it turned on. While playing lately, the same problem occure in my PC and still is not turning on until now.. Can someone tell me what's the problem and give me a solution to this? Need answers ASAP!!