Computer can not start properly.

I bought a computer 2 years ago and did not face any problem till date. But now it has started to give me some problems.
I do not understand at this moment the reason or cause for this problem. That is, when I start my computer, it does not open.
I checked the power supply and every thing. My RAM and other cables are also ok.
Please make me understand, what is my computer's problem?
