"Call a really good locksmith."
OK, I'm here.
Does the key turn? If not, try flooding the keyway with WD-40. Run the key in and out a few times. Then put the key in and rap the side of the padlock that the key cuts face (that is, the side that the jagged part of the key faces) HARD with a hammer. The key may turn after that.
If the key turns but the lock doesn't open, try putting a large screwdriver through the shackle and prying WHILE THE KEY IS TURNED. Or, rap the top (where the shackle goes in) of the lock with a hammer while the key is turned.
If you can, post a brand and description of the lock and the specific problem. Maybe we can be of more help.