Check your config.php file.
Code:
/*--------------------------------------------------*\
| MuOnline Database Connection Settings |
| |
| $core['db_host'] : Database host address |
| $core['db_name'] : Database name |
| $core['db_use'] : SQL Authentication user |
| $core['db_password'] : SQL Authentication password |
\*--------------------------------------------------*/
$core['db_host'] = "127.0.0.1";
$core['db_name'] = "MuOnline";
$core['db_user']= "sa";
$core['db_password'] = "---------;
Make sure to look closely at
$core['db_host'] = "127.0.0.1";
if 127.0.0.1 doesn't work, try (local)
If not, try your SQL server name. You might also want to check if your ODBC connections are all fine.