It seems that a key to a locking file cabinet is one of those things that is always around, until you need it. They are inevitably missing when you need to lock or unlock the cabinet. The bigger problem is of course when you have a locked cabinet and need to get in. Fear not, there is an effective method for opening a file cabinet with a pop out lock on it.
Take the ruler and place it on the top of the file cabinet. Align it so that one edge of the ruler is perpendicular to the centre of the lock face. This should put the straightedge on the centre line of the lock cylinder assembly. Measure and mark a spot on this line that is 5 cm (2 inches) from the front edge of the cabinet.
Drill a 3 mm (1/8 inch) hole on this mark through the cabinet.
Peer through the hole using a small flashlight, and try and see the plunger bolt on the lock. Don't worry if you cannot see it. Continue to the next step.
Put the end of the ice pick in the hole. Angle the handle of the pick in order to manoeuvre the tip to the plunger bolt you found earlier. If you did not see it, move the tip around until you feel it, the hole you drilled is right in line with it.
Once you feel the pick against the plunger bolt, angle it to exert force on the plunger bolt and the lock will pop open. You can now open the drawers and can remove the lock assembly if you wish to replace it.
Repair the small hole in your cabinet with a little wood or metal putty and a dab of paint.