Try it with the passive speakers plugged in to Spk2 sockets. If they work now then this points to a knackered Mute relay.
If they still don't work then you could have a blown a fuse on the power amp board. This is the board which is vertical and runs along the back of the unit just inboard of the heat sinks. There are two fuses towards the end of the board near the speaker outlets.
Good Luck