Do you mean that you need to re-join ClearPass to the domain, or your clients? If the clients, they fail all at the same time?
In case you need to re-join ClearPass, please run the 'ad testjoin' command (see here). What could have happened, is that for some reason the Computer object for ClearPass that is created during the join is removed or disabled. I have seen AD admins or change management tools being responsible for this as they might not recognize the computer object and just remove/disable it.
