I was manually deleting some users yesterday and everything was fine until a few minutes after i restarted the server. Now i cant login as ANYTHING! not root, not a user, not into FTP, not to send mail, not into cpanel or whm.

I have a feeling i may have done something to the /etc/passwd file. Does this mean i will have to reformat? I have backups on a seperate drive, but i dont want to format until last resort :(

What error do you get when logging in? You may be able to rescue it with some fiddling around with lilo etc, but it's tricky. Search for "reset root password linux"

Boot into single user mode and look for a mistake in your password file...