Unfortunately, deleting the Photos Library can only be done on the Mac, not on a phone.
So, to be clear, is it true that when you look at Settings>General>iPhone Storage that, in addition to that cute graph, you see the list that includes Photos, and it says 16 GB next to it? Mine says 4.46 GB, and then Documents and Data says 4.46 GB. I have 15000 pictures, but I have "Optimize Storage" checked.
Have you tried a forced re-start or a factory reset as Matti Haveri suggested? You would want to make sure you were backed up to iCloud, first, I think.
Force restart iPhone - Apple Support (JO)
Reset iPhone settings to their defaults - Apple Support (JO)
Do you still have pictures on iCloud Photos? If there's nothing there that you want to keep, you could turn on iCloud again, wait a few days, and delete everything. Might work... let us know