Just adding my thoughts here;
the bios might be corrupt, or its settings are looking for something that's not there.
Following on from your comment about unplugging everything, while you do that before you turn it back on again try resetting the bios, (Jumper switch, or removing the CMOS battery for ten minutes.)
When a computer turns on it goes through P.O.S.T
(Power on self test)
verify CPU registers
verify the integrity of the BIOS code itself
verify some basic components like DMA, timer, interrupt controller
find, size, and verify system main memory
initialize BIOS
discover, initialize, and catalog all system buses and devices
pass control to other specialized BIOSes (if and when required)
provide a user interface for system's configuration
identify, organize, and select which devices are available for booting
construct whatever system environment that is required by the target operating system
Taken from wikipedia.
During the POST routine, you get a series of beeps to indicate where the problem is, check to make sure you motherboard has a speaker, if not you can buy one from almost anywere online for next to nothing
www. google .com/products/catalog?q=motherboard+speaker&hl=en&safe=off&client=firefox-a&hs=aeE&rls=org.mozilla:en-US:official&prmd=imvns&biw=1280&bih=893&um=1&ie=UTF-8&tbm=shop&cid=12741005373895223740&sa=X&ei=H4VkT46zB47B8gO68rGbCA&ved=0CJMBEPMCMAA
The beep codes you can find here:
http://www. computerhope .com/beep.htm
Please remove the spaces from the domain names in my quotes, as I don't have permission to post URLs yet
hope this helps.