Lawnmover blade stuck

Sounds like you may have sheared the key, which is located under the flywheel. If you are trying to remove the bolt that holds the blade make sure to disconnect the spark plug first. When I have had a stubborn blade, I try and find someone to hold the blade while I either use a good quality wrench or if need be a good vise grip. You can always take the bolt somewhere and find a replacement if you mess it up.