How to break a padlock on a bike?

I was on my way the other day on my bike and the padlock broke. This isn't a basic cheap padlock that anyone can pick, it's an expensive one that breaks itself down the place where you put the key if it's overly forced. I think I hit a bump or something because it's cracked. I have a replacement lock but I need the chain to use the lock with, how can I break the lock off? I don't have massive pliers available, I'm not very strong, and there's only a tiny amount of space between the chain and the actual curve of the lock :( I'm not interested in stealing, I just want my bike available to lock! :(