I would suspect that the host updated your php to version 7.x of which your forum version is not compatible,
I would first log into your hosting cpanel and check if that is in fact true.
Most host will let you roll back to a older php version for a while.
But you probably were on php 5.x
Also while there you may be able to look at the error logs but that error usually means the database can't be found.
So I would check if the database is there and verify the database path, name and database user and database user password
Then compare it to forum/includes/config.inc.php and update that files contents on the top couple of lines with the database info
