Hi there,
This happened to mine - the disc-guides (the plastic bits the discs slide into) had come off their rails so the unit couldn't move them in and out of the laser area.
It also wouldn't eject the caddy as it couldn't move the current disc back into it.
I took the top off the unit and gently pushed the offending ones back onto their tracks and it worked fine.
Worth a try before going the garage route.
Cheers, Matt