Over 2 months ago Twitch announced that all Helix endpoints will require both a Client ID, and an OAuth token. https://discuss.dev.twitch.com/t/requiring-oauth-for-helix-twitch-api-endpoints
Today was the 2nd scheduled window, this one lasting 4 hours, in the rollout of the changes. You got that error because you haven’t updated your code to be ready for those changes, and because of the gradual rollout when the 4 hour window ended your code started working again.