You might have a dead mbo. I had this happened to me not long ago. My pc was working perfectly prior I tore it down to test out a new mbo. After that, I put the mbo back on and it does not boot any more, just led light come and fans come on briefly and stop. For some reasons, it must had been shorted out and killed the mbo. Before this incident, I had removed and installed this same board many times, no problem. Should be note, I did not wear a anti static wrist ban, this maybe been the caused of dead.