Haha, I just had this problem little over a week ago and made a thread about it.

When they do, I'll edit it onto this post. Hopefully Microsoft post a full guide on the problem and the solution. For some reason this is happening a lot recently. If that doesn't work them I'm sorry, I don't know what else to suggest. For me, that didn't work, but what did work was turning the 360 upright.

Clean any dirt and dust off, leave for 10 - 30 mins. Microsoft aren't that helpful, they give you the same ol' solution, Disconnect everything, Including the hard drive. Its pretty serious because if your past you one year warranty, then theres a chance you might have to cough up some cash. Thing s damaged or/and it doesn't recognise the disc. After hours of trying I finally found a solution. just like you, my xbox was fine but I just got home with Resident evil 5 and it would say 'open tray' I tried with many games but none would work. Conincidently enough I had the same problem recently, I sent a message to all my friends and only got a couple of replies.

Original xbox laser cleaning