While a number of issues potentially cause a Brother HL-4070CDW to go offline, one of the most common causes of this issue is an incorrect configuration of the Status Monitor program, as reported by manufacturer Brother. Other causes include networking and power issues.
In many cases, the "Offline" status message that appears in the Status Monitor program for the Brother HL-4070CDW is erroneous, especially in environments where multiple Brother printers are connected to a computer or network. This is due to the Status Monitor program associating the HL-4070CDW's printer driver software with the wrong device on the network, or the correct driver with the wrong physical printer. Status Monitor issues of this kind are resolved by reconfiguring the Device to Monitor and Device to Display settings in the Status Monitor to reflect the correct driver and printer.
If the Status Monitor is correctly configured and the error persists, it is likely that the printer is in fact offline or is having difficulty communicating with the computer running Status Monitor. Checking that the HL-4070CDW is powered on and that it is connected to the computer or network is a good first step to take to resolve this issue. If the printer is powered up and connected, replacing the cable that connects the printer to a computer or network or connecting the printer to a different device are the next steps to take to resolve this issue.