This happens to me fairly often. When I know the musicians or can listen to the sound check, I try to set my volume then to get a decent recording without having to do so.
But, when it does happen, here’s my work path:
First, copy the file so as not to lose the original, that should be number 1 step in any audio editing,
Then run an analysis to get a numeric value on each of the samples, before and after the volume change.
Then run an envelope amplification tool, starting at an inverse of the numeric values above, say for example 60% up to 100% of volume with the transition occurring as close as you can to the section where you lowered the volume.
Alternatively, post the show, in flac and some of us might be willing to give it a go.