Wayne
If you can,try to turn it back down using an ezyout. If not,drop a few drops of tapping fluid to the threads and let it soak abit. Give the fluid time to seep in. Then use the ezyout to get it out.(Up, down, up, down)Im sure it,s just some rust thats built up .
What ever you do,DO NOT TRY TO DRILL IT OUT AND RETAP.Im sure I dont have to tell you why and what will happen.