Try it in a different machine - if it still doesn't open.. it's probably broken.
There should be a little hole on the front of it - this is a release mechanism, just poke a pin in there and the tray should come out. Once out, make sure there is no dust or crap in there and try again.
If it just makes a clunk but never opens, this is a sign that it's time for a new one.
You can get a DVD-R/W for like £12 ($20) now.