Not enough info in your post to know if this is what happened, but if you enter an incorrect code thrice in succession the stereo goes into a lockout mode. I had this happen to me a couple times and I ended up doing as Stew advised, leaving the radio on for 30 or 60 minutes (forget how long) while driving. After enough time elapses you can re-enter the code. Start with that since it's an easy option to eliminate. If you have the owner's manual it might contain an explanation in the stereo section.