1. Have you restarted the machine and then tried to access AD?
2. Is Exchane on the same server as AD? This is not a recommended proctice, it it is, but it does work. If it is on another server, where are you trying to access it from?
3. Any error messages or event log entries? Follow these up and you should be able to discover the fix for the error.
4. As you can see, there is an extraodinary lack of informtion in your question to go on. Please post the whole situation, servers, location af ad error messages or lack thereof, versions, etc., so we have something to go on rather than doing a wastful Q&A session to get the information needed to resolve the problem.