Some rec.'s ave a reset button behind the volume knob. Just pull it off and take a look.
For factory default settings, remove the AC power cord from the wall, while holding the power on button and ? button (see owners manual for the correct button) reinstall the AC power cord, all the display will start blinking and again see the owners manual as all this is from memory from three years ago...
The following is from my AVR-4800 manual
Initialization Of The Microprocessor (see manual index for page).
1) Switch off the unit and remove the AC cord from the AVR-4800 rear panel. Wait for 15 seconds.
2) Press and hold the 6.1/7.1 SURROUND button and DIRECT button at the same time, plug in the AC cord.
3) Check that the entire display is flashing with an interval of about 1 second, and release your fingers from the 2 buttons and the microprocessor will be initialized.
If the microprocessor has been reset, all the button settings are reset to the factory default values (the values set upon shipment from the factory).
If step 3 doesn't work, start over from step 1.
BTW this happen to my father's 3803 with no sub output a few years ago and the 3803 procedure worked, no problems since.
Just have to reprogram everthing from scratch. (Write down your menu settings: speaker size and distance, radio stations, etc. for easier reprograming).
Hope this helps