Exchange Cluster problem. Please help

Dear all,
could somebody please help:
what’s the main components required for Exchange cluster to communicate with Domain Controller.
Cluster is on W2K member server, DC – Windows NT.
We had it set up recently and everything has been workingfine until yesterday. Now users in Domain can logon and access domain resources, but unable to authenticate username on Exchange, therefore – unable to access mailboxes. We’ve checked everything – permissions, trusts – all in place. But even Domain Admins can’t access Exchange server. Error message: “access is denied”.Looks like it’s not talking to DC to authenticate user accounts.
Is there anything I’m missing here to check?
Many thanks.