Good Temporary Fixes:
Repeatedly open/close tray till game works
Tap xbox while it's reading
Lay xbox opposite way you have it (horizontal/vertical; this makes dust fall)
Go to Microsoft for repair (so they can give you a new/same xbox that will get the same problem)
Get new xbox (so you can waste money only to find you'll get the same problem)
Good permanent fixes:
Use CleanDr x disc to clean dust(got mine via gamestop power up rewards coupon;a dusty xbox will have a extremely noisy AC adapter)
Replace lens (if you know it's broke- important signs are if your games are getting scratched or/and the console makes a clicking noise when powering on;WARNING: WILL VOID WARRANTY)
Remove any disks, turn it off, eject tray, force tray close but leave it open a tad, once it closes itself reboot xbox (had to do this once because I left tray open after unplugging it)
Open up xbox to readjust laser lens if it is scratching your games or none of these solutions work(WARNING:WILL VOID WARRENTY)
I've had the original xbox 360 ever since the FF 13 version came out. I clean it every 2 weeks and repair it when something stupid like this happens. I think I know more than microsoft.