You most likely moved or deleted the password file associated with your GUI. Or you might have applied a HAAst update which (until the version released March 2018) would mistakenly remove that file during the automated update (install_updates.sh)
You can recreate the .htpasswd file as described in section 3.4.5 of the installation guide (Secure the Web Interface). I’ll repeat the Linux command here to save you from digging out the manual:
htpasswd -c /usr/local/haast/web_interface/.htpasswd user1
The above line will create username ‘user1’ and then prompt for a password to be associated with the username. To reset the password just repeat the process. Note that the “-c” parameter creates a new file, so it removes other username/password combinations from this directory. If you want multiple usernames for this directory please check the man page of htpasswd for help.
(After March 2018 if you run install_updates.sh to update your version, then your .htpasswd file will be preserved)