Key lock on my 2003 honda civic does not work on drv door

As of last night when trying to open door with key... all of a sudden the key seems as if it is not the correct key to open door. It will not go into the lock and seems stiff or frozen when trying to push it in further and will not turn in either direction. Do you think that by spraying graphite into the lock on the outside can help this problem temporarily? The key does work on the passenger side. I've tried to use the valet key (gray) and it worked two times but it now is acting as the master key (black) does when inserting it into the driver door side...it just won't open. Any ideas???