Problem with remote server seeing main server

My remote server is on a network with the main server and about 30 other computers. All of a sudden yesterday the remote server can't seem to see the server but the connection is still there. We run a program off the main server and it is still working but under network computers I cannot see the main sever. I can still see the other 2 servers and other computers on the network but just not this one server (which works still for the other computers on the system.) Any idea on how I can troubleshoot this problem? I already restarted the remote server and it showed the main server this morning but now an hour and half later its missing again.

HELP ><