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 4.4, but the upgrade script might not have been run yet so the database is still only setup for Version 4.3. 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