The good news is that even if you lost a lot of muscle while dieting down, getting it back is significantly easier than gaining it the first time.
There's still some debate on all the specific mechanisms around it (Satellite cells being deposited and sticking around being one of the leading theories), but the evidence is pretty conclusive that there's 'muscle memory' and significantly less stimulus and time is necessary to get back to prior levels of musculature than getting to new levels, even after years.
There's still some debate on all the specific mechanisms around it (Satellite cells being deposited and sticking around being one of the leading theories), but the evidence is pretty conclusive that there's 'muscle memory' and significantly less stimulus and time is necessary to get back to prior levels of musculature than getting to new levels, even after years.

