You need to set the lock peg back into the track.
Under the tray, you'll see a little track about 1/8" wide. If you look inside under the tray through the front, you'll see a little white plastic peg that is probably out to the left of the track. With a metal ruler, move the peg towards the track groove while slightly pushing up on the back of the tray.
Once it pops in, the tray will move freely again.
CPU64, August 2009