Input 2 will either be the Jack on the front, or the red Phono on the back of the unit.
The Jack is used to Instrument (e.g. guitar or bass) or Mic level inputs.
The Phono is used for Line level inputs (e.g. synthesiser or mixer output).
Ensure that the input 2 switch is in the correct position for your chosen input source.
For stereo Line inputs, ensure that both input 1 and input 2 switches are in the Line position.