1- Is it server or client issue?

a. Client/Server:

i. Cannot find internal resources:

1. Be sure in DNS, dynamic updates are happening (Ipconfig /registerdns will force to register it manually)

ii. Cannot find external resources:

1. Be sure in DNS, root hints are configured

2. Confirm in DNS, Forwarding is set up properly if you use in it

2- Affecting one or more clients? –if problem comes to more than one client maybe we should take a look at networking hard-wares.