Down for Maintenance - An upgrade is currently in progress
Seeing this error indicates that the version of the files do not match the version of your database. So for example the files uploaded might be for Version 7.9, but the upgrade script might not have been run yet so the database is still only setup for Version 7.8. To resolve this you need to do a full reupload of the current versions WHMCS files, and run through any upgrade process that is presented to you when doing that and once completed you will be able to access the system again.
For the correct upgrade procedure, please refer to http://docs.whmcs.com/Upgrading
Critical Error - Unable to connect to the database
This error means your database cannot be connected to. Please check the database connection details in the configuration.php file and update as needed (for example, if you recently changed the password or renamed the database itself). This could also be an issue with MySQL being down or an empty configuration.php file.
If you have confirmed that the database name, user name and password are correct, ensure that the MySQL user has been assigned to the desired database and given full access to it, as this is needed for the connection to work. If you are unsure of how to do this, please contact your hosting provider or server administrator.
Error Message "Field xxx doesn't have a default value" during installation
This error occurs when you try to install WHMCS with MySQL Strict Mode enabled. WHMCS requires strict mode to be disabled.
MySQL server has gone away
This error can often occur when upgrading WHMCS due to the large number of MySQL queries performed, but it can potentially manifest at any time. It indicates that the MySQL Server is closing the connection WHMCS is using to perform the current task. This is usually due to the timeout value being set too low to allow all the upgrade tasks to be completed.
You'd need to work with your server admin/hosting provider to increase the wait_timeout and perhaps also the max_allowed_packet values in the MySQL Configuration.
If the problem is occurring during generation operation, then the issue should now be resolved with the increased limits. If performing an upgrade one should restore pre-upgrade database backups before running the upgrade script again.