How to diagnose?

We're running SBS2003 with Exchange, SQL Server and ISA Server. I
can't make it in to the office for a couple days. I'm trying to fix
from home (ain't gonna happen). I can connect to the VPN just fine and
access files that are on the server. SQL Server is accessible as well.
Exchange is not working. We are receiving no emails. We can exchange
emails within the LAN and I've been told emails are going out as well.
I was able to use Remote Desktop to connect to the server and I
rebooted it. Now I can't remote into it anymore, but can still connect
to the VPN, etc. Haven't been able to connect to webmail for a few
days.

I'm assuming the problem is within ISA Server, but I'm not sure how to
figure out what's happening or where to look. Any suggestions would be
appreciated. I'm going in to the office tomorrow if I can't fix it
remotely.