Because I am unaware how large your network is and the amount of clients relying on the Active Directory service,...
you might need to try this during non-business hours. Active Directory contains a special recovery/safe mode, which can be used to troubleshoot such problems or perform operations that wouldn't be possible during normal operation. During startup you'll be presented with an option to press F8 to bring up the DOS-like menu. From there you can select the appropriate menu item to troubleshoot your Active Directory.
Combining this "feature" with Microsoft's support page, I'm sure you'll be able to find a solution. In the case you've also got a backup of your server (and Active Directory), you might be able to use it during a repair or restore process, but this will depend on a lot of other parameters that you can find on Microsoft's page.
In closing, check your event viewer log for any reported conflicts or errors and use their event ID when searching through the above mentioned support page.
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