One channel is not working

We'll definitely need more info such as brand and model of amplifier and how it's installed. If the amp is new, first place to look would be the inputs. You can swap the L and R inputs to see if the issue is upstream of the amp. If it's a used amp, it's possible one of the channels is damaged and would need to be repaired. If swapping the inputs causes the channels to switch (i.e. the non-working channel changes), that would indicate the amp is fine and the problem is in the source or the signal wiring between the source and the amp. Also check the amp's settings to make sure it's set up to run the way you need it to. Look for input channel switches and bridge mode switches. These settings will change which channels have output.