Access Lost on SOME clients

I have a group of Engineers physically connected to 4 hubs. Logically they are in the same workgroup. Today, they cannot get email, connect to the internet or access some of the file servers on the network. There are users in different logical groups connected to the same hubs who are not affected. I’ve checked everything I can think of (ip addresses, as we use DHCP), tried changing the affected machines to a different workgroup… Does anyone have an idea where to check next. or what could cause this baffling situation?