Computer will not boot…again

I hope you guys can help me with this ! I’m about to go insane ! Here goes : computer will not boot, monitor stays on stand by but all lights and fans work. This is a recurring problem so i’m 99.9% sure that nothing fried. Last time this happened was after i cleaned the interior with a can of compressed air, this time i installed a sound card (yes i tested without it and with the old one before and after clearing the c-mos). Anything that i might have overlooked big and small please tell me.

P.s. Last time it worked after having done nothing specific except maybe for having had cleared the cmos for the 100th time.

Thanks in advance guys.