computer wont start

first of all, if this post is in the wrong place, im sorry. couldnt find any other place =/

anyway...

today i got my wreck to computer back from the repair. what i did the second after i plugged it in was to start it up, remove the old graphiccard drivers, and then turn of the computer and put in the new one, and then put on the computer again and install the new drivers. everything worked pretty fine this far. went online for some time, when my computer all of a sudden froze. "wtf?" i thought, but i had to go shop so i just turned of the computer. when i got home i instantly tried to turn on the computer again. didnt work the first time.. didnt work the second time.. bingo, third times the charm. went online again, and just checked around in forums and stuff like that, when suddenly it froze again. and i cant turn it on since then. the problem i had before (so i had to send it for repair) was back, but i dont understand how.

what could be the problem? my old problem was that everything in the computer started, but the computer itself didnt start (if that makes sense). the motherboard was dead. but that is replaced now, and shouldnt be defect from just using it like that.

anyone that might have any idea what the problem could be? i want a functional computer =(