My window sensor just fell off the window and broke. How do I get a new one?
If your sensor is under warranty I would call support. If it is not under warranty, I would buy one on amazon.com.
But when you say it broke, if it just popped apart, you can probably put it back together and remount it.
AFAIK, if you have professional monitoring, all devices are protected under warranty. I would call Ring to see if this is the case.