The router has been replaced or the WiFi password has been changed.
Set up the WiFi connection again as follows:
Press and hold the WIFI button on the remote control included with the robot vacuum cleaner until PaIR appears on the robot vacuum cleaner display.
The login details for the home WiFi network are reset.
Complete the pairing process for your robot vacuum cleaner in the app again so that the WiFi connection to the home network can be re-established.
The robot vacuum cleaner is then shown as a connected appliance in the app again.