What did you change that might cause this? It sounds like the ST to Amazon integration got screwed up (either the ARN number from Lambda to the developer site or the Oauth and token from the App to the Lambda code). Have you checked that? To be clear, when you are telling Alexa to turn on the hue lights are you actually using the app (i.e. Alexa, tell smartthings…) or simply “Alexa, turn on/off…”…If it is the latter, that does NOT use my app, going back to the code issues I mentioned.
Let me know if that helps at all.