Cannot access accounts when no password
I am using High Sierra (10.13.2), and usually run my computer without a logon password (secure location). I now find that I cannot get into my user account to make changes or to change my password - System Preferences give a rejection shake when I try. Generally, other things such as logging on to my account or installing software, work just fine without a password.
Why has this just started with High Sierra, and how can I get past this to make changes to my user account or change my password?
Thank, Paul.
iMac, macOS High Sierra (10.13.2), i7 2.8Ghz, 12 GB RAM