This does happen sometimes because of how easy it is to damage a cardboard ticket. From our experience, it is very easy to resolve this – just go to a ticket desk at the station and ask the staff member to check your ticket. If it does not work properly, they will replace it for you free of charge.
Remember to always greet staff with ‘Bonjour!’ first. In France, it is considered very rude to just go straight into asking for something without greeting first.