Mine did this last night. I found the following procedure on a youtube video somewhere. Can't find the link because YouTube is blocked here:

1: Turn off the machine (with no disc in).

2: Turn on the machine by opening the disc tray.

3: Physically force the disc tray back in with force. Not so much that you snap the thing or knock it off its runners, but enough that the motors stop retracting the tray.

The tray should stop ajar for a few seconds before the machine composes itself and continues to retract the tray.

4: Reboot the Xbox and the drive should function normally.

I'm unsure exactly why this works, but my best tech head says that the controller for the drive gets into a bit of a tizzy and doesn't realize that the laser is incorrectly calibrated. Forcing the tray this way makes the drive perform some sort of self-check, in which it re-calibrates itself and works.