Front end login has stopped working

People are unable to login to the front end. It was working!
They get a white page with "You are not authorised to view this page"
Note, I also have Membership Pro installed.
Oddly I can access the site using others credentials from my PC.
I have spent days on this so far - can anyone help?

Sounds like you turned login off. Within CB > Configuration > General ensure "Login Method" is configured correctly. Re-save configuration if it is to be sure the configuration has saved.

Login Method is set to Username (I changed it to Email then changed it back)
Under CMS "Users Allow user registration" is set to "Off"
I have just tried to Register another dummy user and I get "You are not authorised to view this page" on pressing the Submit Button

For registration you've Joomlas turned off. You need to ensure CBs is turned on within CB > Configuration > Registration by setting "Allow User Registration" to 'Yes, Independent of CMS "allow registration"'. Check that CB Core within CB > Plugin Management is published and public access. Next in Extensions > Plugins ensure "Authentication - Joomla" is published and public. Alternative is you're forcing HTTPS for login, but are trying to login from HTTP. Edit your CB login module within Extensions > Modules and under "Login and Logout Options" set "Encrypt Login Form" to "No".

www.joomlapolis.com/documentation/127-community-builder\