My screen goes black

Hello,
last night i was using my computer.then my cousin brought over his cpu,then we disconnected mine and connected his cpu to my moniter after a while we put everything back the way it was.the next day i turned on my computer but it wasnt working it just went black after the start up(blue) screen after that, nothing.I then disconnected my cpu and reconnected it to my old moniter, but i found out that it was my cpu that wasnt working not my moniters so i rebooted my cpu and still nothing happened. it wont show the WELCOME sign anymore i need help and fast!!! i dont know whats wrong with it i opened it up and cleared all the dust,I put it in safe mode but it wont show nothing it just goes black. i checked all the power outlets everything works fine its just the cpu it doesnt work anymore i just wanna know if there is someone out there that can help me solve my problem PLEASE!!!




P.S my computer is a windows xp
it is also only about 4 years old