I don't have a solution but I also have this same issue.
I have a Fender BXR 300 combo with two inputs. The first input stopped working recently but has absolutely no sound coming through. 2nd input works great.
I've owned this amp for 15 years and it has spent most of that time unused and in storage. Been using it regularly the last few months when all of a sudden one input stops working.
I found this from the BXR instruction manual.
(A) INPUT: Two inputs are provided;
Input 1 - High gain
Input 2 - Low gain
The high gain input (INPUT 1) should be used for most
instruments. This input can handle as much as 3.5 volts
R.M.S., while the low gain input (INPUT 2) accepts 7 volts
R.M.S. without audible distortion. For both cases this is
greater than any active bass with a 9 volt battery can
produce. if you are using a bass with dual 9 volt batteries, or
a "hot" passive bass it might be wise to try input 2. For the
rare case where two players want to use one amplifier, or
where two instruments are to be used during the
performance, both inputs may be used.