Open a locked file cabinet without a key by tilting up the front of the cabinet and pushing up the lever located near the front on the bottom of the cabinet. The lever will release the lock and the cabinet will open. You can also break the lock with a screwdriver or power drill if you cannot located the lever.
2 Additional Answers
If you have an inexpensive file cabinet, I would try using a paper clip to unlock the file cabinet. If that doesn't work, you may need to call a locksmith to unlock it.
To open a locked file cabinet without a key you are going to need some tools. You will need to drill out the lock. Once the lock is drilled out you will need a crowbar to open the drawers.