As your manual tells you, under SERVICE you'll find the CLEAR SETUP function. This brings the unit back to factory defults without erasing your presets.
Or you can try a full reinitialization by pressing and holding the 8 key at power on. You may want to wait until you see 4000 in the display to press the 8 key.
This function will erase your presets.
Whatever procedure you will use, it's always a good thing to backup your presets before!