The card has probably developed an error. Flash cards (including SD cards) have a limited number of writes, and, while the expected number is large, sometimes failures happen early. Or it could simply be a manufacturing defect. In either of these cases, Sandisk — which has awesome long warranty periods ­— will probably replace the card, but that doesn't do any good for your older photos.

You should definitely stop using it now. Writing more data may make recovery harder, but, also, you may lose the whole card. If you don't have a backup one and care about photos you make take on the rest of vacation, it's worth paying tourist prices for a new SD card.

That said, the next step is to put the card into a computer and see if you can access the files there. Of course, it's nicest to do this as soon as possible, but if you don't have a computer easily accessible while traveling, put the card in a safe place and check it when you get home. That will make it clear if it's a card problem or a camera problem. Even if the pictures can be copied off okay (whew!), I wouldn't trust the card again.

Also in the future: don't leave all of your eggs in the collection basket. When I'm traveling, I make sure to copy everything to my laptop at the end of the day, and if possible where I am, then upload those photos to cloud backup nightly. If that's not possible, you may want to look at some of the solutions suggested in this question about travel backups.