I’ve had a pair of Tradfri lights working well through Home Assistant for most of the past year. But, the remote used most often started blinking red- indicating that the battery needs replacing.
According to Ikea, just replace the battery. Done.
But, in Home Assistant, nothing is ever that simple.
So, I had to reset the light and the remote to “factory”, then reconnect the light and remote to the Gateway. Now the remote and the Ikea app control the light. But in Home Assistant, that light is “Unavailable”.
The other light is still working through Home Assistant, so I know that Home Assistant is talking to the Gateway.
Edit:
All is working now. I restarted Home Assistant and discovered that the entity name was now different. Edited my groups.yaml config file (the only place that entity was used), and everything works again.