You could also try to phone the central authority for fines issued on DB trains (which is located in Baden-Baden) - but be warned, they usually don't know what they are talking about (having worked with DB as someone who checks tickets for several years, I know what I'm talking about
). It is also important whether you were fined on a regular metro or u-bahn/tramway train (the authority responsible for fines issued there is the local transport authority) or in a DB train (RegionalExpress, RegionalBahn or S-Bahn). In the latter case, Baden-Baden might help you, otherwise you should try to contact the local transportation authority. I know that it's quite confusing, but in germany, the different local transport companies (and there's lots of them) are solely responsible for ticket checks and the collection of fines on their respective lines - collaboration is seldom happening.
And don't worry - your passport or anything like that won't be invalidated if you don't pay your fine - riding a means of public transport without a ticket is an "Ordnungswidrigkeit" (something like a "minor crime") and won't effect the validity of your visa or anything the like. However, there's the possibility that the transport company will file a suit for "Erschleichen von Dienstleistungen" which is indeed a crime. But that'll usually only happen if you get caught several times within a short period of time.