Common problem - transmission and gear
Often with veteran bicycles - especially of our sort - the situation is simple:
the gear and switcher with it's outer parts gets comrpomised.
Whether the cable being ripped off in pedestrian traffick as - for instance in Budapest - you don't have enough bicycle lockers, hotspots to leave your bike without chaining it against a pole for example,
or that something or somebody pulls it out...
To fix this is relatively simple, but to get original parts if some were lost on the way are very hard to get.