Hello. My name is Michael. I can help you with your question.
It is always possible that it has, indeed, expired. I'm assuming that you checked that?
It is also possible that their systems were down or that one of their credit clearinghouse connections were down. From gas stations, they are often connected via satellite and if there is weather moving in, it is possible for them to either lose a connection or have an intermittent connection, which will give bogus error messages.
You can call the card provider - they will probably tell you they don't see a denied transaction. If that's the case, then the above applies and you have nothing to worry about. It's rare, but it does happen.