I disassembled an entire PC, motherboard,RAM,Hard Drive, etc, all taken out but the processor. I used an air compressor to take the dust off. After I was sure it was off, I used an electronic contact cleaner while holding the motherboard in a vertical position, spraying on it and waiting for the liquid to fall down. After that, I assembled it once again.
Also, I used the air compressor to clean the power source.
After it was all assembled, I tried turning the PC on....and it didn't.
Now, this was an old PC, but it was functioning before I started. It was assembled in 1998, the cables (in Spanish the technical name is "bus de datos" but I have no idea how it is called in English, but they were IDE not SATA) were IDE.

Try using another spare motherboard and test it out. I have a hunch that your motherboard might be damage. Just a hunch, did you make sure the motherboard was dry before you re-assembled your PC?

Could be a short, try taking everything out again and putting it back in, it will be a pain but it's worth a try