The main reason that all of the sensors and the home-hub can go offline, is if the home-hub has no power. So making sure it is still plugged in, and powered on is the best first step.
If the home-hub is connected and powered on, but everything is still showing offline, it may mean that connectivity is poor or the internet is down. You can check this on the EE coverage website, to see if there is a signal issue affecting your area.
Here you can put in your postcode, and check if there are any reported problems in the area with the signal. If there are, this will likely be the cause of the issue. And if that is the case you need to wait, the home-hub will go back online automatically once the 4G signal is back up.
If it doesn't come back on automatically after a 4G outage, you can try unplugging and then re-plugging the home-hub to the power. This will manually restart the home-hub, making it recheck all of its connections in the process, which should mean it reconnects automatically.
If you have tried all of these things and all of the sensors, including the home-hub, are showing as offline in the app, we would recommend getting in touch with our support team, who will be able to help you further.