@Technicolor,

I would start with simply replugging the devices. It sounds as though the connection over your network to your HomeKit/Matter Hub has been broken/interrupted - even though the device is still connected to the Wi-Fi and controllable through the Tapo app. You could consider rebooting your Controller, but is not recommended if you have other devices connected.

Do you happen to have an IoT network or have isolated any devices on your network? Your Matter Controller and Devices should be on the same network and they will not function if one is isolated from the other. 

What does your network topology look like? Since it is a network level protocol, misconfigured networks (such as Dual NAT or having two routers) have a much higher chance of interfering with normal operation.

If none of the above applies, you should reset and add the device to the Controller Again. If the device doesn't connect, you may need to remove it from your Apple Home before setting it up again.