Having trouble opening door with the key, key won't go in

I lock mine before I get out, as soon as I open the door and only use the key when unlocking the door to open the door to get back in. pick up a tube of powder graphite at Walmart or a parts store,squeeze a little powder in the key slot and or just put a little on the key,working the key in/out/turning it in the key slot,see what happens.$2.00-$4.00....