MacBook Pro 3.1 internal (replacement) LCD not detected by graphics card; external monitor OK
This sounds like a hardware issue, but there might be a few things you can try.

I would boot the mac with the cmd+v keys to show a verbose output. If this doesn't work, you could try resetting the NVRAM/PRAM (docs can be easily found on the apple site)

If you don't see the Internal Display in the about my mac app, the Display Data cable might be improperly seated. There is a fairly good tutorial on iFixit.com http://www.ifixit.com/Guide/Installing-MacBook-Pro-15-Inch-Core-2-Duo-Models-A1226-and-A1260-Display-Data-Cable/1312/1