Quick answer: in general, if you are holding a railpass and there's no mandatory reservation for the next train and it accepts your railpass, it doesn't matter anyway - just hop on the next train (you may get no seat if you had a reservation before).

If the train line has a mandatory reservation, you'll need to buy a new one paying the normal reservation fee (the money paid before is lost) and the next train does need to have free places available; if it’s normal public transport and not a rail replacement you are not allowed to just take the next train because of a delayed bus or tram or underground.

It actually depends. If it's a rail replacement bus, then normal passengers’ rights apply. But without details (route, date, departure time), only general advice is possible.