Connecting to outside server

[Guest_imported]

I have a few machines on a small network which connect from one Windows 2000 server to another via an ISDN line. This worked perfectly for a while but now I can't get to the remote machine. I can ping it, tracert it and Telnet to it OK but it won't connect via it's IP address. We used to map a drive to it OK but it now won't do this. The server will connect to the remote server perfectly OK. This has happened on a couple of machines now and I also have another couple of machines that won't connect to the home server also which were perfectly OK previously. The machines are running Novell to an older server and Windows networking to the new server then out via ISDN to the remote server. We are not that technically brilliant so any help you can give us would be appreciated - in basic English please!