Can anyone help with a strange and sudden problem

FB 1.5.0.4306
Windows 200 Server

This system has run happily for several months. Suddenly this
morning it began to go very slow. I stopped and started the FB
service and can no longer connect.

Using IBOConsole Communication diagnostics, it passes the
Communication Diagnostics test where server = 'server', port = 3050
(or gds_db).

But if I try to log in to the server I get

ISC ERROR CODE:335544721

ISC ERROR MESSAGE:
Unable to complete network request to host "server".
Failed to establish a connection.

This is nothing to do with the database file - I don't even get that
far.

I reinstalled. Rebooted. Reinstalled again. No joy.

I have kept the client going by installing FB on a workstation, and
using that as the server for the moment, and this works fine - though
a bit slow.

BTW, in case this is relevant, I am using a VPN and Terminal Services
to maintain the server.

TIA for any clues.

Roger Vellacott.