Ignition key won't turn

The ignition key would not turn when I tried to start up the car yesterday. I tried the spare key and it was the same. It just stuck firm in the ignition. After some time it suddenly freed up all by itself. This has happened once before, but did not take so long to resolve. I am worried in case it happens on a long journey as I am driving to Devon next weekend and take a few stops on the way. What can I do if it won't free up at all???