Was it scheduled, or no? if scheduled, did you follow FB's requirements? (like connecting to stream more than 10 minutes in advance)
Were you patient and aware speed of light isn't that fast, and especially not free FB streaming servers, so your live content and the preview may have a 20-30 second or more delay/lag? I've noticed the FB live producer preview lag is often worse than a cell connected tablet.. so ymmv
For me, the preview stops on occasion, but the stream itself is alive and well. I always monitor from another device (or have someone doing that for me). If the preview stops, I found simply hitting Refresh in the browser restarts the preview
Also, side note, on Win10 v2004, latest Edge browser in Private mode, I've noticed the FB live stream comments don't update, but they do in Chrome... go figure. Also, trimming videos and updating video Thumbnails works well with Chrome, but not latest Edge (FB website doesn't even offer some features to Edge)