Lots of Discos out there are prone to water leaks in the windshield, sunroof and roof rack areas. Water has its way of finding electrical components! My disco has an amplifier under the right side kick panel. Located behind the golve box area bolted on the lower right side A pillar (where lots of leaks evidently like to flow). I think there is a 7.5 amp fuse in the factory unit. Check that and the connector for corrosion. If this amp is out or unplugged it will do what you described.
** as posted above by Joey, always always check the fuse boxes first before doing any other kind of exploritory surgery!!!
If you take power away from the receiver, you will need your stereo security code handy or it will be of less use to you than it is now (you don't even get to change the station!).