First you need to determine the model of your karaoke machine. No background for the old model 900A may be 1. No disk, 2. Misplaced DVD, 900A does not have the program to decompress DVD images, or 3. VCD drive failure.
For the new model 900C/X or 900P, it may be 1. DVD ROM failure, 2. Video decompression board failure, or 3. No background when no disk is placed, the hard disk will automatically play the built-in background. If it is none of the above, then it is a hard disk failure. If you cannot troubleshoot the problem, please send it to a Service Center.