Pick up a scratch awl and coat the shaft with a spray or oil lubricant. This will reduce any sparking and help you punch out the lock.
Insert the tip of the scratch awl into the keyhole until it can go no further.
Strike the scratch awl with a hammer repeatedly until you punch through the back of the file cabinet lock. You will have punched through the back of the lock when you no longer meet resistance.
Pull the scratch awl out of the lock once you've penetrated through the back of it and set the awl aside.
Insert a flat-head screwdriver into the punched-out file cabinet lock and turn the screwdriver clockwise. If you can't turn the screwdriver by hand, cinch vice grips over the handle of the screwdriver and turn clockwise. The lock will disengage and you can pull the drawer open.