Contact the manufacturer of the file cabinet and request a replacement key.
Take the code number from the original key (hopefully you wrote it down and kept it somewhere safe) to a real locksmith, not just a key cutter. He should be able to make a replacement key for your file cabinet.
Put on safety goggles. If you're desperate to remove a lock from a file cabinet, stick a blade screwdriver between the lock and the cabinet. Hit it with a hammer to drive it in, then pry. Pry around all the sides until you can get the lock to pop out or quit working.
Put on safety goggles and use a pair of bolt cutters to cut the shackle of the padlock or combination lock. Remove the lock.
Put on safety goggles and use a crowbar to pry the hasp off the file cabinet if you can't cut the lock and you're desperate.
Slide the flat end of the crowbar underneath the hasp. Push down slowly on the other end until the hasp is free from the file cabinet.