Urgent, site down MySQL error

[Bat21]

My site went down at 00:35 U.K. time and the server is showing MySQL down. An attempted restart of the MySQL service is showing the following error;-
/usr/sbin/mysqld: File '/var/lib/mysql/mysql-bin-log/mysql_bin.index' not found (Errcode: 2 - No such file or directory)

What has caused this please?

I am running v4.2.5

[Mark.B]

This is something you are best speaking to your host about, as it is a server issue rather than a vBulletin issue.

[Bat21]

This is something you are best speaking to your host about, as it is a server issue rather than a vBulletin issue.

Thank you Mark.
We have just this second resolved this. For some reason the mysql directory got renamed to some random name. It is a dedicated server so I'm not sure what happened there. Will investigate later.