Domain conroller is down not connect user to Application

We use windows 2003 R2 and our one application is running on that system and all user use that server ..

Also get Error RPC services is not available..

After restart server its start and the problem is resolved but i can’t restart server all time..

i show the error report of event viewer the problem is show the server could not connect to domain server..he loss the connection domain server ..

Why this problem is getting plz help me