Is there a disc in it? If the drive has trouble reading from it it may lock up the drive.
In that case restarting and pressing the eject button repeatedly may open the tray.
If that doesn't work just use an eject stick. That's a small metal stick which usually comes with the drive. Just push it in the hole under the tray and it should open.
It worked fine for the first two openings w/ the straightened paper clip. Then it started making "expensive" noises. So I replaced with an LG 22X DVD re-writer from Best Buy (OK , I was impatient).