That specific error means that MyBB can't load the required MySQL extension. This may be because your web-host has removed it from your server.
However, you could try the following:
1) If you login to your ftp account and navigate to the folder "inc" and then find the file called "config.php"
2) Download that file to your computer and then open it in a text editor such as
Notepad++.
3) With the file opened up,
find the following line:
Code:
$config['database']['type']
If it
matches the following:
Code:
$config['database']['type'] = 'mysqli';
replace it with:
Code:
$config['database']['type'] = 'mysql';
OR
if it
matches the following:
Code:
$config['database']['type'] = 'mysql';
replace it with:
Code:
$config['database']['type'] = 'mysqli';
4) Save the file and then re-upload it,
replacing the existing config.php file that is on your server.
If the above doesn't work, you will need to contact your web-host, quoting the error MyBB returned in order to resolve it.