As for why the app stopped working today for a lot of people, official statement from Athom in another topic:
Philips hue - broken on 5.5.8 update
Emile
Signify, the company behind Philips Hue, suddenly added rate limits without letting their partners (us) know.We updated the Hue app very quickly to avoid the rate limits. Unfortunately, because this automatically restarts the app, and many users already hit the limits in their previous app session, the new app session couldn’t find the hue bridge. Well, shit.
However, the new app is compatible with the new rate limits. Within 24h everything should be back to normal.
…
