Dirty pot, bad solder joint, transistor going out and etc. but hopefully it is just dirty if it has a pot if not then the next best thing is a bad solder joint. After that it can get a little more involved. While it is off turn the volume control up and down if it has a pot, if not and it does have a warranty don't open it and send or take it to a authorized tech ONLY. Good luck.
Randy
Randy