The printers in your store are wired into a network router using an ethernet cable. This router might also be broadcasting a wifi signal that your devices use to connect to the internet.
In order for Toast receipts and kitchen orders to print successfully the Toast device needs to be connected to the same network as your printers at all times. If at any time a device is on a different network than the printer, the printing jobs will begin to fail.
The Toast printing error screen has been updated so that now, when your print job fails, it will offer you a list of networks that you should try to connect to, as well as showing you a default button that will take you to the network settings so that you can connect to the correct network.
Remember that the printing error may not be the reason that the printer does not work, but this is definitely the most common error. If you are connected to the correct network, more trouble shooting may be required.