First welcome, and sorry you’re having issues. But, honestly asking ‘what could be wrong’?
Sunspots? Hippos? New neighbor moved in and lit up a new wifi network? (#3 and maybe #2 could happen) No clue. We need a lot more to go on.
Fortunately the community built a great zigbee guide and a great zigbee troubleshooting guide here as part of the cookbook…
Scroll down to both zigbee and troubleshooting (zigbee has its own troubleshooting section)