Windows Active Directory backup to a file
Solution 1:
You should always have more than one domain controller to help avoid this!
Microsoft provides documentation for this task. For recent versions, you can use the windows server backup tool to perform an unscheduled backup and then later a full restore: https://technet.microsoft.com/en-us/library/cc753359(v=ws.10).aspx
You will want to read through the dozens of notes and cautions before you start. I would also check DNS very carefully after.