I don't know if this is any use to you, but I had the problem where I would put a game in, it would spin for a moment, then say 'open tray' as if there was no game in it.
1.I got it working again by opening the tray, then pushing it shut again (without a game in it) it would make a little noise as if the laser was re-aligning or something. Then I'd put the game in and it would work.
This only worked for about a week though (and sometimes Id have to push it in a few times before it would work again)
2. After that stopped working I opened the xbox up and opened the DVD bit. Behind the laser (on the board the laser is mounted on) there are 2 screws, I turned the left screw (looking at the head of the screws face on) about a half turn to the right, then it started working again, it has worked since although that was only about a month ago so we shall see how long that lasts.
If you keep looking around the web you'll find some suggestions somewhere, it took me a good hour or two search before I found these two, and if your xbox is past its warranty you either keep trying yourself or pay someone to do it(haha).
My xbox is an old white arcade one, think its about 3 years old(perhaps 4) so don't know if the model and age makes a difference to the way to fix these problems.