this may sound really stupidly simple but make sure that you heyboard is connected properly!!

my family computer would do anthing once and i looked at it and the keyboard wasnt connected!!


also another idea is to remove as much harware as possible so keep the psu,mobo,1stick of ram,no graphics card if you have onboard and so on, then try booting and see what happens, then i fit works add peices in one at a time, may fix your problem it worked for me once just ripping it apart and putting it back together bit by bit, i dont know how that works it just does.