Blocked up or jamed.If key not too worn and lock just went stiff,spray some oil in the key hole gently twist key back n forth a little then spray more oil in,repeat lots if this frees it get some spray greas in all locks to prevent rest jaming,req lota work to replace lock and the lock has to be biult to a key unless you find lock and key set for sale,
Steven,s, January 2012