I am sure, or hope, you are aware that installing Exchange on a Domain Controller is not recommended. This could be the root of the problem, but I would get on a call with Microsoft support to verify it.
I would check your event logs and look for errors pertaining to authentication. Plus you have not said whether any recent updates were applied to either Exchange or the DC, or any changes were made at all. MS can help you sort this one out, it is too complicated a question I think for a good answer here.