Never created a password yet locked out

Never created a password for xp home edition, yet after
using for many months, tonight I got a "Unlock Computer"
screen saying that "this computer is in use and has been
locked......only[lists user names] can unlock this
computer"..........then I'm to type in a user name and
password.......since I have never created a password what
do I do? Also, why did this happen in the first place?

Thank you for any help.