Hi jackbn and Welcome to The Forum.
Can't say why this may have happened, but something to check. In the Start Menu Search or a Run... window, type netplwiz which will open the userpasswords2 window.
Highlight your user name.
If ticked, un-tick the "Users must enter a password to use this computer. Click apply, you'll be prompted to add a password and verify said password. This is entirely optional, you can just OK your way out.
HTH Elmer.