If you lost the key to your locked filing cabinet and you need to get into it, you can call a locksmith and pay to have a new key made, or you can drill the lock out to get the cabinet unlocked. Obviously drilling the cabinet will leave the lock inoperable, but it will get you into you filing cabinet.
Place a 1/8-inch drill bit into your hand drill and place the tip of the drill bit on the center of the keyhole on the lock.
Drill a hole straight through the lock. If the drill bit starts to smoke, Spray it with a little water from a water bottle to keep it cool. The locks used on filing cabinets are made of relatively soft metal, so burning the bit should not be a problem.
Place the blade of a screwdriver into the hole and attempt to turn the lock counterclockwise. If the lock turns, it will unlock the filing cabinet. If the lock does not turn, increase the size of the drill bit and repeat steps 2 and 3 until the lock does turn.