If the drive were the victim of a bad flash (not an improper one, a bad one) it most likely wouldn't work at all, it would be 'bricked'.
If the key is missing or wrong it's not going to play any game discs at all, but it will still play CDs and DVD movies just fine.
If it was flashed by someone that didn't have a clue what they were doing then the key could possibly be gone for good, say they just straight flashed a hacked FW to the drive without dumping the drives FW first and making a backup, in that case the key is gone, forever. You'd have to dump the drives FW and see if there was a key there to even start troubleshooting that aspect of it, and even if a key is there that's still no guarantee it's the right one and there's no way to cross reference it against anything on the 360 to tell.
If the thing plays DVD movies fine it could even be the laser as well, they can be picky to a point of reading DVDs but not wanting to touch a game disc. If ya have a known good drive there swap lasers between them and see what results ya get, the Samsung and Hitachi use the same one, so the drive doesn't have to be the same model/version at all to try that out.