Sub-servers no longer resolving

So I woke up this morning and went to log into one of my sub-servers running web analytics and found out that the sub server wasn’t resolving. I then tried all my various sub servers and none of them were working (even ftp.domain.com wasn’t working). The tlds resolved without a problem.

Does anyone have any suggestions of where I should start looking to resolve this issue?

Resolved it, BIND DNS wasn’t enabled.