Lock Screen Black on 16.3

You could be in Do Not Disturb focus, or it could be that the photo you used is not compatible. If you upgraded from iOS15 to iOS16.3 that will definitely be the case.

I advise you delete that Lock Screen pair and start again

Personalize your iPhone Lock Screen – Apple Support (UK)

This has also happened to me; both my lock screen and my Home Screen turn black, this even though after turning off my iPhone and rebooting multiple times, using the same photos that I have always used successfully up to iOS 16.2. Contrary to the suggestion of replier LD150 in this thread, I do not have the opportunity to use Do Not Disturb Focus in these circumstances, and I can hardly believe Apple would make it so I cannot use my favorite photos for basic applications between iOS 16.2 and 16.3. This looks like a bug that needs to be fixed.

If it was a bug we would all have this problem and we don't.

Force restart it and if changing to another photo dies not work then backup, restore to factory and restore from backup.