laptop/ dual monitor question

[Bluff Daddy]

I have been running a monitor off of my laptop so I can 4 table and it was working fine, then the other day I unhooked it to plug it in to my cpu and when I hooked it back up it wont work anymore. The little light goes from green to yellow. I tried to call toshiba but they wont help me b/c I dont have the serial # which rubbed off the bottom and isnt readable. I mean why they hell would you put a serial # that can just be rubbed off on there? So anyways im trying to find the damn reciept now but figured Id ask here if anyone knows what the problem might be. Ive tried everything I can think of messed with all the settings and troubleshooting.