Heavily depends on the train/company.

If it's a train with mandatory reservations expect a 100-300€ fine and/or having to leave at the next stop. It is often not possible to get to the platform without a reservation (ticket barriers).

If it's a train with optional reservations you're allowed to board and can sit on an untaken seat. In exceptional cases you may not be allowed to board if the train is too full.

Which route and date are you looking at ? There are always alternatives if trains with mandatory reservations are sold out.

We are going to another city so the problem is solved. Thank you for answering!