VPN connectivity issue

VPN connectivity between two windows servers failed, it was working fine till yesterday. error806: The vpn connection between your computer and the vpn server could not be completed. The most common causefor this failure is that at least one internet device(for example, a firewall or a router) between your computer and the VPN server is not configured to allow Generic Routing Encapsulation (GRE) protocol packets. If the problem persists contact your network administrator or internet service provider.