Standard debugging is to pull parts until it posts. A failed memory, PCI card, etc. can cause these symptoms. Power off, open case pull video -- does it beep? no. bummer, power off, pull memory, power on, does it beep, no bummer, pull next thing like power and data cable to disk. Eventually you get down to just the MB and that will work (or it won't). If just MB and PSU don't post then try a new PSU -- its more likely to fail than MB.
Here is a great ref, its for new build but you can adapt to your current problem. http://www.tomshardware.com/forum/261145-31-perform-steps-posting-boot-video-problems
oh, almost forgot. Once you pull a piece and the PC posts, try putting it back in. If it got loose int eh socket or just got some rust at the contact reseating the card or memory or cable is sometimes all you need to do to fix it.