Scheduled or ad-hoc Video event?
Did you use FB's /live/producer and see if preview video appears?
I've had FB settings change for unknown reasons (with volunteers, no idea if accident, FB screw-up, or ???). As such, as we only use Scheduled Events (for consistent URL, visible to non-FB users), as we use /live/producer to monitor stream and act as digital usher, I routinely check ALL video settings to make sure something unexpected hasn't changed. And once there is an issue with a Scheduled Event, getting re-connected can be a real hassle (granted, haven't bothered with any recent (terrible) FB's documentation) to point of often not working at all.
My recommendation - start with FB's /Live/Producer page. RTFM (make sure to change to streaming software, copy key, start streaming... does preview window populate? do not 'Pass Go' until you resolve.. presuming no change in OBS settings (you did check that first, right?). no issue with OS user profile, or OBS Profile? Assuming OBS settings as they should be, then diving deep into FB video event settings is next step
And side note... FB's Scheduled Video auto-start is NOT reliable for us... and hasn't been most of '23. Sometimes Live video will start in 15-20 seconds, and will for weeks on end, then it won't. We either manually start at Scheduled time to be safe, or wait 20 seconds, and then swiftly turn off Auto-Start and manually hit go Live... (as our automation needs to capture an intro screen to use as Thumbnail, before time sequence change of scenes for 1st 5 minutes of livestream before resuming manual OBS control of Scene selection