How to unlock a bike lock when the key is missing?

I lost the key to my bike lock. It's a standard U-shaped metal lock, but I couldn't find any more information about it. When asked at a hardware store, they said they couldn't make a new key and that the lock will have to be broken. Can anyone suggest how to go about breaking this lock? I hate to see my own bike locked up on the backyard without being used.

bolt cutters work real good and if you can't get them a hack saw will work in time.

A little time with a hack saw

BOLT CUTTERS will work just fine if you cant get a hold of a par try asking a friend or rent one at your local hardware store

if it is a master pad lock get a pry bar and a large crescet wrench put the pry bar thru the ring on the lock and the crescent wrench on the base of the lock and twist it like it was a nut on a bolt works like a charm everytime