If it wasnt modded give M$ a ring I'm sure they can repair it for you - was it one of the 1st release xbox's as there is a well known problem with them and M$ will repair it even if the warrenty has gone.
Anyways, give Microsoft a call, They usually charge around £30-£50 for a repair.
If you want to give it a shot yourself (version 1.0 XBox only, very easy though), try lik-sang (don't buy from lik-sang europe as it works out more expensive)