Help Please – Possible dead Motherboard?

I recently swapped my motherboard for a known bad to test etc… Confirmed that MB dead. Put my motherboard back in and now BIOS wont boot – no video display – have checked power supply, RAM, Video cards (3). No POST. Just dead??? Any suggestions???(Friend suggested possible short on case???) I’m lost. – pls help – now rather urgent.

Vaughan.