optical disk drive will not burn disks?

I have a Mac Powerbook G4. It´s 5 years old. A dog knocked it on the floor 18 months ago, and it has worked perfectly up til now.
When I try to burn a disk it cancels the burn process stating the device would not work. I´ve had the problem before, but it always worked soon after a few tries. Now nothing. I have always used the same cd-r disks. Can this be repaired cheaply or do I have to replace it with a new drive. The sound when I insert a blank to burn is whirring vibrating noise, then the fan starts and then it´s like the motor switching off, and up pops the reject message. Help would be much appreciated.