Master Locks are used by many to secure their property. Master locks are considered to be the master of all locks. They can be purchased at most hardware and retail stores. They can lock with a combination, key or both. Never lose the combination or key to a master lock. Never drill into a lock because this can damage the drill, drill bit and the lock. Always let a professional locksmith drill the lock if required.
Check to see if you have a backup plan. When you got the Master Lock, did you place the key somewhere for safekeeping? If you received this lock from work or school, it's a good chance they have a key for the lock.
Go to a local hardware store and order another key for the Master Lock. It may be able to get a replacement key only if you wrote down the number that was printed on the original key. Without this number there is no way of knowing which key will open your lock.
Cut the lock with bolt cutters. Place the bolt cutters on the part of the Master Lock that is attached to the object and cut there. If this doesn't work, you can always try picking it.
Insert a small pick into the chamber of the Master Lock. Then insert the small Allen wrench into the chamber and rub the teeth of the lock, starting from the inside rubbing out. If this doesn't work, there is always the professional.
Call a locksmith. It will probably be expensive, but he can remove the lock for you. He also may be able to make you another key. Most locksmiths will require that you have a picture ID and some proof that the locked item belongs to you. You can find a locksmith in the phone book or by doing an online search for your postcode.