VPN wont ReConnect

ADSL connection using a Draytek 2600 router with built in VPN server.

I recently had a successful VPN connection with a friend, through this
connection I was able to use VNC to assist him sort out a problem.

As he needed to reboot we naturally lost the connection, on trying to
reconnect he showed up on my router as connected, his computer showed
a successful connection but I was unable to do anything.

I was not able to conect using VNC neither was I able to ping him on
the address shown on my router as allocated to him.

I'm suspecting this is a cache problem but hasanyone any ideas what
the problem may be.

Geoff Lane