The two main areas of erectile tissue in the penis, the corpus cavernosa, are like sponges - they are not simply empty spaces. I believe that if you have no erections for some time, the tissues that make up all the spongy stuff start to stick together. This makes it more difficult to achieve good erections... so they stick together more... etc.
The pain you may feel when having a good erection after this may be due to stretching and even minor tearing of some of those tissues.
I imagine that to reverse this you would need to somehow get more blood pressure into the penis than usual - maybe with a penis pump, maybe with jelqing; I don't know.
In any case, however it works, the "use it or lose it" opinion seems to be pretty widely held.
The pain you may feel when having a good erection after this may be due to stretching and even minor tearing of some of those tissues.
I imagine that to reverse this you would need to somehow get more blood pressure into the penis than usual - maybe with a penis pump, maybe with jelqing; I don't know.
In any case, however it works, the "use it or lose it" opinion seems to be pretty widely held.