It could be any number of things.

There could be a short in the wiring of the guitar somewhere, it could be a bad cable or bad wiring in the amp. The pickups on your guitar could be picking up interference from some sort of electric equipment nearby, like TVs or computers. The speaker could be shot.

If the buzzing from the amp stops when you put your hand on the strings or any other metal part, then there's probably an electrical problem with the guitar.

Just try to isolate the problem.