black screen after apple logo please help

My family gave me a macbook pro 15 inch laptop. they bought it back in aug 2007. Everything has worked fine they told me until my sister had gotten into a small fender bender. But it was enough to send the laptop flying into some stuff in the back of the car, and dent the bottom of it. the problem is that the lap top starts up with gray screen and apple logo, then goes black, but the computer is still running. Also, if i hook it up to a external monitor, it works perfectly, as in it shows up on the external monitor. the dent is located on the botoom right hand corner if your looking at the computer head on. Also the computer gets hotter than usual. Any ideas on how to fix it? I'm a PC guy usually but would like to delve into macs also, so i would like to get this working. I'm running lepoard snow. Thats all i really know about it, any suggestions? I took it into the apple store and they said the logic board could be shorting out because of the aluminum hitting the logic board where the dent is. but if that was true, why would i get a display in the first place? any ideas.

Thank you for suggesting anyting, because it would help.

Since your MBP works OK when you attach an external monitor, that would indicate a problem with either the internal display or the video cable. Both fixes require a disassembly of the machine and considerable $$. Since it's a 2007 model and has been given a hard knock, (dented) it would not be covered by the Apple care extended warranty (if you have it). You have to decide whether or not you want to spend money to repair it.

Regards.

if it is either or of the things u mentioned, do you think it will be easy to diagnose? as in i wiggled the cables, and all and it didnt work. looked visually and it looked ok. also, if that was the case, then why does it start up with the picutre of the apple and the spinning wheel, but then goes black when it goes into the system itself?

I was referring to the internal video cable not the external. It could well be that initially when you first turn on the machine, the screen will light but then goes black as soon as it warms (which is fairly quick).

Do this: Don't attach your external monitor. Turn on the MBP and allow it to go black. Turn off all the lights around you and look closely at the screen. Can you see any images at all? Post back the results.

We can eliminate the video processor (GPU) because video shows up on the external monitor. However, it could still be the internal cable that's loose from the connector. Something is happening as soon as the machine warms.

Regards.