I get this from time to time and it's either the SD card not all the way in and needs to be pulled out and pushed back in or the connection between the hm and Pi may needs to be resettled. But, I'm pretty sure if nothing has changed in your router and Pi configuration, it's the SD card.
We have some new features in the 4.24 case that we haven't unveiled yet that will effectively end this possibility in the future.