1. Ping the VPN server to ensure it is available. To ping the server, select the “Start” menu and “Run.” Then, in the “Open” text field, enter “Ping” followed by the VPN server name or IP address followed by the “Enter” button. If the server is not available, you will see this displayed on the DOS console screen.
2. Confirm your computer's firewall settings are still configured to permit traffic to and from your VPN. If you are using L2TP for the VPN tunnel, open your firewall configuration panel and ensure UDP port 1701 is not blocked. If it is, change the firewall setting to permit UDP traffic through port 1701. The specific menus to select for port configuration of your computer's firewall will vary depending on the vendor and version installed on your computer.

3. Confirm that the firewall settings will allow traffic on UDP ports 500 and 4500 if using IKEv2 for the VPN tunnel.
4. Reinstall the client-side and secure root server key certificates on your computer that were provided by your network administrator at the time of setup for the VPN. The method of installation will depend on the type of VPN software installed on your computer.
5. Request a verification of the VPN server security certificate if you still cannot connect to your VPN and receive VPN Error 800.