How to sign into Mac Air when I can't remember my sign in password

I can't remember my p/w for computer. I wrote it down but it isn't working. I followed instructions of entering a wrong p/w 3X's but the msg to enter apple id & p/w doesn't come up. The other suggestions don't help because there are no boxes in upper L if I can't enter my p/w. Is there anything else I can do without erasing everything on computer and starting over?