If you've lost the key or forgotten the combination to your bicycle U-lock, padlock or front door, you may have to resort to breaking the lock open. The method you use will depend on the strength and type of the lock, as well as how much damage you are prepared to do, either to the lock or to the item it is attached to. Once you've broken the lock open, it is unlikely you'll be able to use it again, so have a good look around for the keys or code before taking this step.
Get your jack from the boot of your car.
Spray the lock with compressed air, or preferably liquid nitrogen, if you can get ahold of it. Wait until the liquid nitrogen has finished bubbling. The metal should now be brittle enough to break.
Put the jack through the U-lock in its unexpanded condition and start to turn, or pump, the handle.
Keep turning the handle until the U-lock breaks open.
Try to cut the padlock open with a small hacksaw. If the metal is too tough for this, move on to Step 2.
Spray the lock with liquid nitrogen and wait until it has stopped bubbling.
Hit the padlock hard with a large hammer. This should be enough to break it.
Give the door a good hard kick where the lock is. This is sometimes enough to pop open a weak Yale lock.
Get a cordless power drill.
Start the drill in the widest part of the key hole and press hard to drill on through. This will break apart the pins inside the lock and the lock will open.
If a padlock is being used to hold a chain together as a security measure, rather than attempt to hacksaw through the padlock, it is usually better to try and saw through a link in the chain. If you can cut through the one nearest to the padlock and leave most of the length of the chain undamaged, you may also be able to use it again. A padlock, however, would be too damaged to use again once it has been cut through.
Only attempt to break open your own locks, or locks of friends and family with their permission. Trying to break open security measures on other people's property constitutes a crime with severe penalties.