If you are having thi skind of problem, usually is due to electric problem, memory problem, hard drive problem or bios corruption, also may be done by an incompatible device attached to your system.
1 and most important, allways check and update your bios, download the floppy version or the usb version and install it.
2 Remove all cards and additional devices you may have attached to your pc and try start it again, if it works, start inserting one by one to see which one is the incompatible one..
3 if none of this works, remove one of your rams and start the machine then do the same with the other to see if one of your memories is faulty, also if you installed the new motherboard with memories you were using with a prior motherboard, they may not be the correct ones for your new one, read the manual for that to see the proper ones for it.
4 If you have another hard drive, remove the one you are using and install teh new one to see if you can at least get into the bios, if it works, try installing something in this other hard drive to see if it works properly.
Do as mention here step by steb one by one to ISOLATE th eproblem.
I forgot to mention, the power supply could be damaged or the cables not well attached, check that first of all and be sure there are not screws touching the motherboard, specially from below where is dificult to see them.