If the Mac was dropped, the ? indicates that the disk was probably damage in the drop if not more then that.
If I were doing this I would take out the disk and test it first since it is most sensitive to a drop.
Here are possible causes:
- Corrupt or failed hard drive.
- Critical system file(s) are damaged.
- Some other hardware failure.
I would first boot into Safe Mode then repair the hard drive and permissions. Restart normally. If this doesn't resolve the problem, then reinstall OS X. If this doesn't resolve the problem, then repartition and reformat the hard drive and reinstall OS X. If this doesn't resolve the problem, replace the hard drive.
I cannot get the computer to start up with absolutely anything. No disc will allow me to start up to check Disk Utility, or to re-install the OS - and I can't run AHT.
All I see is the gray screen, period. :/