It depends on if it is a vehicle or house door. Many times the lock needs to be lubricated and it will start working properly. Some times the key is worn down and a 'code key' would need to be cut. If it is a Toyota it could be that the split wafers are bad. If it is a Honda sidewinder then it could also be a bad wafer. Need more information.
You can open a key lock on a door knob, or you can open a deadbolt lock on the door itself.
Because your key is SEN
Lock
Surely there is a key lock on the door where you can insert the key and open the door.
You'll need a professional locksmith to open the lock without damaging the lock and door .
lock smith . take hinges off the door or take the door knob and plates off the door
You can open a locked door (From the outside: With a key)(From the inside: With the lock opener, usually by twisting it) :-)
Insert key in the door lock and turn to unlock the door.
Unlock it from inside the car with the door lock.
open a door with a lock in the area you obtaned the key in
With your key in the door lock cylinder.
Open the door, child lock will be visibale by door catch. Use car key in 'key hole' to turn from lock to unlock and back again