I needed to re-add one of my Tradfri switches: TRÅDFRI Kabelloser Dimmer, weiß - IKEA Deutschland
I don´t use the switch within the Phoscon App, but use inputs for HomeAssistant. Means there are no groups to activate lights within the webapp.
Since I have added the switch again and push the button once, all lights switch on, but just virtual and not the bulbs itselfs.
2.13.04 / 18.12.2021
Click here in order to open the Old WEB App:
There you can find the groups of your On/Off switch and assign lights to it.
Or you can just query all the Groups from the Rest API using for instance YARC
Here my switch is group 20 and I can tell by the lights which one.
You will be able to identify the Group number in Home Assistance, which might have changed if you re-included it.
You might also consider making a dedicated group in PWA and use that for control from HA instead as that will work even if the remote runs out of battery.