Your boiler is still heating and providing hot water, but it has lost its connection to your home WiFi, so it cannot be controlled or monitored remotely through the app.

DIY fix possible

Check that your WiFi router is switched on and working, and that other devices in your home can get online.Confirm that the WiFi network name and password entered in the boiler or app settings are still correct, especially if you have recently changed your router or password.Restart your router and wait a few minutes, then restart the boiler by pressing and holding the reset button or switching the boiler off and on at the isolator.If the code remains, move your router closer to the boiler or check the boiler's WiFi signal strength in its settings, then re-enter the network details or repeat the connection process in the Viessmann app.