A newbe question about missing a connection

If the next train doesn’t require a reservation, then you can just jump on. If you had a reservation for the train you missed, then you can get a refund from the place where you booked it. Or you ask the station staff to rebook you, which is of course only possible if the next train has reservations.

If the next train requires a reservation, then ask the staff at the station to rebook you. This should normally be free of charge.

Also see:

https://www.seat61.com/european-train-travel.htm#if-things-go-wrong

You say it’s a key journey - can you clarify what you mean by that?

If it’s key because there is only one a day for example, then you have to think about what it does to your plans if you arrive a day later.

If it’s key because reservations are mandatory then you also have to consider what happens if reservations on later trains are already sold out - specifically in this context, if it’s a Eurostar to or from London.

If you give details of the route you are thinking about people can provide more detailed answers.