The most simple check is to see if Home Assistant is able to see all the registered entities and if they are all reported as available.
Usually, when a device stops responding, is not paired or it goes down or any other reason; Home Assistant is not going to delete those entries. The devices and entities do not disappear just because the device stops communicating with Home Assistant. This allows Home Assistant to report problems in devices that it expects to communicate with that are now unavailable.
To see the unavailable entities go to Settings => Devices & Services => Entities. Then click on the "Status" column to reorder. The unavailable ones will have a red circle.
You might also find entities that have a red circular arrow as in the image below. Those are working now, but have been recently unavailable and it would be good to review why was that so.
When something is not working either because the devices are physicially not doing what they should or because Home Assistant is reporting them as unavailable, first we need to evaluate what is the extension of the problem which can also explain what exactly is down.
In that file go to the "data" section and you will find the "host" field, which will have either a domain name (uses DNS configured in the raspberry Pi) or an IP:
And now you have to re-add the device. For that in the bottom of the page there is a big "+ ADD INTEGRATION" button. Click on it and choose "ESPHome", then you will see:
where you will have to introduce the IP address of the device you just removed.Do this with each of the affected devices.
If all ESPHome devices are down: TODO
Zigbee devices: This integration controls the ConBee II connected devices (the lights for instance).
If some of the Zigbee devices are down: First identify the affected devices (just in the same way as explained for the ESPHome devices).We need to reconnect them. For this we don't need to delete the devices first, we just directly click on +ADD INTEGRATION button and we click on "Add Zigbee device". This automatically enters an active search for Zigbee devices. This search doesn't go forever so you will from time to time have to click in a "Search Again" button.During this time, the controller devices of the Zigbee network that are not working have to be put in pairing mode. For this the controller, that has a DC input of 5V, has to be desconnected and connected to the power supply 5 times. That makes them enter in pairing mode and that HAS TO HAPPEN ideally while the search for new devices is actively happening in the Home Assistant interface. Then the device will show up and automatically configure itself (it will ask to optionally assign it a Pod area but that's not relevant to have the device working again).
If all the Zigbee devices stop working: TODO.