Http://www.daileyint.com/hmdpc/manual.htm You need to do a systematic check, from wall unit to HDD, simply follows the steps in the manuals. http://www.overclockers.com/help-my-pc-wont-boot-troubleshooting-guide/
Please find two really good, links to assist you to troubleshoot and repair your PC. Both sites have step by step diagrams to follow to work thorough all the possibilities.

Well that most likely means the video on the motherboard has gone. Basically you would need a new motherboard.

The price to buy a new board and pay for the installation would be a decent amount of money. You should start looking for a new computer.

One thing you may want to try before you decide to junk it is to take out the RAM and switch the spots. If it has more than one chip then try taking one out and booting it and then swapping it to see if one of the chips are bad.