Have you installed a new head unit or is it still the standard radio that came with the car?
If you have fitted a new one, you could have left the mute cable for car kits loose which uses ground to mute the radio. If it hits something metal and grounds, then it will mute your radio.
If its the OEM radio, the same applies but this would probably only happen if someone has fitted a car kit in the past and hasn't taped up the mute wire or the ISO Loom has been damaged.