It's possible that something could be stuck in the drive somewhere, causing it to think there is a disc jammed, which may cause it to open. How are you at removing and installing hardware? I would be tempted to remove the drive, while it is open, if you can, and give it a gentle shake, in the hope that if there is a foreign object in there, it will drop out.
At worst, if you need a new drive, they are only about fifteen quid, from the likes of Dabs.