If you need to open a locked file cabinet and you have lost or misplaced the key, there are several different options you may want to consider. Some of the methods for doing this can be destructive, and include drilling the lock out, or prying the drawer open. If you do not wish to damage the lock or cabinet, then you will need to contact a locksmith or pick the lock yourself. A locksmith may be able to cut a new key for you by examining the lock for a code number, or pick the lock if no code number is present. Many filing cabinets use simple wafer locks though, which can often be picked by using a paperclip or even rocking a different key up and down while attempting to turn it.
The different methods of gaining access to a locked file cabinet each present benefits in certain situations. Your options may be limited based on factors such as the construction of the cabinet, type of lock, or the desire to avoid damaging either one. If you plan on disposing of the cabinet and lock, and simply want to access the contents, prying a drawer open is sometimes an option. Many filing cabinets are made of thin sheet metal that can be easily bent by a little pressure from a pry bar. This method requires bending the cabinet or drawer so that you gain access to the locking mechanism, which must also be bent out of the way.
Another method of gaining access to a locked file cabinet is drilling out the lock. The lock cylinder will need to be replaced afterwards, but you will typically avoid damaging the cabinet itself. You should typically begin the procedure with a drill bit that is slightly wider than the keyhole and attempt to drill through the pins. If you succeed, it will be possible to open the lock with a screwdriver. Otherwise you will need to use one or more larger bits to drill through the entire lock cylinder; be sure to wear safety goggles while doing this.
Professional locksmiths also have a few ways of dealing with locked file cabinets, so if you do not mind spending some money that is also an option. If your lock has a code number printed on the face of the lock or the bezel, a locksmith may be able to use that to cut you a new key. Locksmiths also have the necessary tools to pick such locks very quickly. Alternatively, by contacting the manufacturer of the cabinet, you may be able to obtain a new key or instructions for bypassing the lock.
Many file cabinets use wafer locks, so you may also want to consider picking it yourself. In order to pick these locks, all you need to do is push each wafer all the way up. One way to accomplish this is to insert a key for a different wafer lock and rock it up and down while you attempt to turn it. Any other thin piece of metal can also do the job, including a paperclip, nail file or knife. To gain access to a locked file cabinet that uses a more complex lock, you will typically need to purchase a professional lock picking kit.