Hyper V server 2008 guest no access to outside network

I successfully setup hyper V server on a test machine and install some guest OS on it. All of it work well with out any problem. But suddenly I lost connectivity to outside world from guest OS. Now I can ping to host OS (Hyper V Server), host to all guest and ping between guests. But none of them have access to outside world.

I was using external virtual network and connect it to physical NIC. All of the guest OSes are using this external Virtual network.

I search a lot on web. But there is no useful info. Please help me to clear the problem.