Some times i cant start up my computer....?

my problem is when i used my computer then shutting them properly when next time i used it sometimes it wont work, i pressed the shut on on then light are up (light means that its already working) then when i check the monitor it doest show anything and its always by luck to work my computer.. is it in my cpu or is it kind of virus.... (i think the monitor is not the problem here cause its brand new) when i start my cpu the cd-rom drive light keeps on blinking (not usuall)....