In recent posts, I've described ten things I take to class. Of the ten, the one I never go to class without (assuming that some technology is going to be used) is a "Plan B".
It's usually literally written on the back of a used envelope (I collect them for that purpose) or else on a piece of paper pulled out of the recycle bin. Occasionally, it's only there in my head, but it's often the very last thing going through my mind before I step into a classroom.
It's never very complete. And it doesn't have to be. Nor does it have to be a complete alternative lesson, together with photocopies.
What you want is something, half an idea is often enough, that will get you over the initial shock of being up the proverbial creek if the YouTube video has been removed, or if there's no Internet connection at all.
After that, experience will kick in and if you limit the amount of technology the lesson needs, you'll at least survive to the end of it and your learners will still have been learning something if that half idea was half a decent one.