Urgent Help Needed!!!

I have ran into a big problem with my site.

I am getting this error…

[B]Fatal error[/B]: Call to undefined function: mysqli_connect() in [B]/home/XXXXX/public_html/XXXXX/XXXX/core/db_mysql41.php[/B] on line [B]25[/B]

I got this problem after receiving a sudden 500 internal server error. My fixed that problem but now im getting this… I was working fine before i got that 500 error.

All the setting that connects to the database seems fine… Someone please help me with this… thanks.