Can't access FreePBX GUI after reboot

Recently performed a new install of FreePBX 12 64 Bit. All was fine until a reboot, after which the FreePBX GUI was no longer accessible. Instead the following error is displayed. Any ideas how to resolve?

FATAL ERROR
SELECT username, password_sha1, extension_low, extension_high, deptname, sections FROM ampusers WHERE username = ''

DB Error: unknown error
Trace Back/var/www/html/admin/libraries/ampuser.class.php:54 die_freepbx()

[0]: SELECT username, password_sha1, extension_low, extension_high,
deptname, sections FROM ampusers WHERE username = ''

DB Error: unknown error

/var/www/html/admin/libraries/ampuser.class.php:13 ampuser->getAmpUser()

/var/www/html/admin/libraries/gui_auth.php:46 ampuser->ampuser()

/var/www/html/admin/bootstrap.php:160 require()

/var/www/html/admin/libraries/gui_auth.php
/etc/freepbx.conf:9 require_once()
/var/www/html/admin/bootstrap.php
/var/www/html/admin/config.php:99 include_once()
/etc/freepbx.conf

“unknown error” is often an indication that one or more of your database files is corrupted or crashed. There is a “repair” command you can run from mysql (log in as the admin) and clean up the database files, assuming you can connect.

If that doesn’t fix it, check your MySQL installation. Something might have gone sideways there for you.

/etc/init.d/mysqld status

?

/etc/init.d/mysqld restart

Get error?
File /var/lib/mysql/mysql.sock exists?

Thanks for the replies. Ended up rebuilding the PBX installation as I was trying to create a Virtual template.