The most likely cause of this is that the database details in your /admin/includes/config.php file need correcting.
Please check the db details in this file to make sure they are correct for your current hosting environment.
The lines to look for are:
$GLOBALS["AL_CFG"]["dbUser"] = 'username';
$GLOBALS["AL_CFG"]["dbPass"] = 'password';
$GLOBALS["AL_CFG"]["dbDatabase"] = 'database';
$GLOBALS["AL_CFG"]["dbServer"] = 'server';
Example values are provided in the lines above, actual config values will be different.
