Acquiring a new key
Take in a key if you have one to your locksmith to duplicate it.
Take in the lock to the locksmith if you do not have a spare key. If the lock is attached to an immovable object, ask the locksmith to come to you. The locksmith may be able to give you a master key or make you a key from the lock design.
Have your locksmith re-key your lock. Re-keying involves changing the tumblers in the lock and giving you a new key. This method is cheaper then buying a new lock, and allows you to keep the old lock.
Write to the manufacturer of the lock if the locksmith can not produce a key. The manufacturer may be able to send you a duplicate or master key based on the serial number of the lock. If all these steps fail you may consider replacing the lock.