All it needs is oil, apply some to the moving parts and wiggle them around to work it back where you can't apply it directly. Even brand new locks work better after a drop of oil on the important bits. Don't go after it with acid, the rust that's binding your lock is buried where you can't get the acid, and even if it did manage to wick back into the lock and do its business, you'll never be able to get it all out. If it's frozen, apply some thin oil like 3 in 1 or Marvel Mystery oil and let it sit for a little bit so it can seep back into the crevices. Then start wiggling things around until the frozen parts start moving again. Careful leverage and creative bashing are helpful for seriously bound up things. Some people find the uttering of loosening incantations to be helpful when things get really stubborn.