If all computers lose access to internet, you have a problem with the modem, router, or ISP (Internet Service Provider) in most cases. Keep in mind that computers do not have to have the internet to work on the internal network. See All Computers Cannot Connect Access the Network for further troubleshooting if you do not have local network connection.
Call the ISP to make sure you have an internet connection to the building.
Power cycle (turn off and back on) the modem, router, and switch. Turn them off in any order, but turn them on in this order: Modem, router, and switch.
After power-cycling all the network devices, take note of the lights on the devices. Amber, red, or no lights at all are usually signs of something wrong.
If the above doesn’t work, reboot each workstation one by one. If it still doesn’t work, you need to get a technician on-site to further diagnose.