Usually, laptop boards don’t require an internal display to boot, but you’d better to reconnect it to see if it gives something new (assuming, of cource, that you didn’t throw it away). If nothing helps, try to disconnect BIOS battery (usually it’s CR2 battery), wait for a while so that it could drain all energy stored at capacitors as leftovers, connect everything back again (including internal display if possible) and turn on. If that didn’t help, then you might have damaged your MoBo physically somehow, and the only way is to either forget about it of give it to reputable repair technician and see if he finds something (and fix it if he/she can).