| Does it make a beeping noise? I'll just post this all, when a computer has a hardware problem and wont start but the light on the MB comes on the computer will beep a certain pattern and the pattern identifies the problem, check it out:
Beep Code: -> Description of Problem:
No Beeps -> Short, No power, Bad CPU/MB, Loose Peripherals
One Beep -> Everything is normal and Computer POSTed fine
Two Beeps -> POST/CMOS Error
One Long Beep, One Short Beep -> Motherboard Problem
One Long Beep, Two Short Beeps -> Video Problem
One Long Beep, Three Short Beeps-> Video Problem
Three Long Beeps -> Keyboard Error
Repeated Long Beeps -> Memory Error
Continuous Hi-Lo Beeps -> CPU Overheating
If you hear nothing as shown above it's the motherboard, cpu or something is loose inside the computer case, it could also be a short circuit or the power supply isnt putting out power to the motherboard, but this last one seems unlikely as you are seeing motherboard lights.
If you hear beeps count them and compare to the list above, in all cases i have to say you are going to have to replace some hardware inside the computer. | 
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