Sounds like to me ya gotta little salt/calcium build up in the threads. Take a wash rag and place it over the turn key. Pair of pliers and apply even pressure to release. Might not be a bad idea to soak it overnight in some Simple Green with water before hand. My guess is that if it's that difficult to get open you're gonna find other issues inside. Gimme a call if I can help...Dip 713 412 1769
X2 Dipsey's advice. Soak it first. After soaking at least several hours in warm water and green I would tap lightly around the cover edges and try to turn the handle. I would not put very much pressure on the handle.
Try dipping that side into some boiling water for a minute or so. That should free up the salt build up. Once you get it open make sure you place a little grease on the threads to prevent it from happening again.