First guess would be to also restart the client machines.
And maybe you call an administrator on the phone who will guide you?
Actually this is not something that you should solve or try to solve alone if it exceeds your competence.
Based on the small amount of information you are giving it is not easy to give good advice.
In any case you need the "Administrator" password of the server.
Attach a physical screen, keyboard and mouse to it.
You need to hit Control-Alt-Delete for the login. If it is not ready for this consider restarting or resetting it until it is. If you cannot get to this point, there might be something really broken on the server.
Login and open a command prompt.
In the command prompt type this:
Computer managagement pops up and on the left there is a menu tree. First find the event logs and check for anything suspicious. You can also check hardware there in the device manager in the same menu tree.
Open another console and try to ping the client machines, for example if the client machine is running and it has the IP 10.10.10.1:
ping -t 10.10.10.1
You can interrupt the ping with control-c
As long as the ping does not succeed check the network connection (cards, cables, routers, switches). Also check if the windows software firewall is enabled and should not be be enabled. Maybe it has been swithced on by an automatic update. But be sure to ask the Administrator about this first. (Edit: of course it would be also possible to configure the firewall so it allows the traffic you need, but if you do not need the firewall at all, because you are behind a router-firewall, chances are switching it off is the best.)
You can also ping your router and gateway. If you need to know the router and gateway IP type "ipconfig" in a console and you can see all the important IP addresses.