Q: Mac does not regonize my password

I bought my Imac in Janurary this year.  This is the third time my password has not worked, yet I have not changed it.

BEfore I kept putting it in until it worked.  Today, it didn't.

I haven't changed it.  It is simple and one I have used for years so I haven't forgotten it.

It's short so it's easy to put in.

I noticed on line there are many posts on various sites about this issue.