Accounts locked on server

Hello,

I have a Mac Mini server running Yosemite, and for some time now have had problems with accounts being disabled. As of today, every single account has been disabled and I can only boot up into single user mode. I seem to recall that there was a way of deleting the global security policy but for the life of me can't remember how. I have searched the forums where I remember looking previously but can't find the commands to disable/delete the security policy. The last time this happened I was able to log in once the policy was deleted.

Can anyone point me in the right direction?

Regards,

Andy.