What brand? Is the lock hooked to the bike? I think Kryptonite will sell keys, but are somewhat specific with the procedure so that you can't just buy keys for locks you don't own.

Anyway, contact the lock manufacturer.

A good locksmith could probably open your lock, and may be able to make a key if they have the blanks available. Whether it is worth the trouble depends on whether it is a $20 lock or a $100 lock.