For whatever reason, networks go down. This is usually due to a failed switch or a server going down. The router and modem going out will indicate no internet, but not loss of internal network connectivity, see All Computers Cannot Access the Internet for further troubleshooting. This is because all the workstations rely on the server which is on the same switch as them so communication does not require a router or modem.
To Resolve: Edit
1. On the server, Run - ncpa.cpl - Does it say "Network Cable Unplugged" on the NIC status? If it does, does it say this on all workstations? If so, the switch is the culprit.
2. If it doesn't say anything about the cabling, reboot the server AND the switch.
3. After they both come back up, if you still don't have local network access you will need to send a tech on-site for further diagnosing.