See if the guidance in this
http://www.tomshardware.com/forum/261145-31-perform-steps-posting-post-boot-video-problems is something you can use.
Standard debugging is to pull parts until the system POSTs then add parts back in until you have isolated failing part. This problem is likely hardware not software. Generally it helps to post specs, model number, rough age and what lead up to the problem.
Eventually it'll turn out to be the power supply, but it could be anything so follow the strategy of pulling parts until you have only the MB, CPU and Power supply left. If it doesn't post then replace the PSU first.
Good luck.