Alright,
here is some step u can do :
1. Did u try to change input cable (phone to amp) ?
2. Did u try to plug output jack to speaker or any other earphone, is it working stereo?
3. What is ur amp? is it still waranty, if yes you can claim for warranty before you open it.
4. If your warranty is gone, you can open the amplifier, then you check the connection between input and output.
Thanks.