I have a single character, A blood elf Mage, named Apollinius, that will not enter the game. When AI attempt to enter the game with him the screen is stuck at the bottom of the core way with no land in sight just a light blue screen. Then I get a WoW 51900319 disconnected message. This only happens with this one character. Other Characters load fine so far.
Character stuck might help move to a safe spot if staying out of the game does not work for at least 30 minutes.
https://us.support.blizzard.com/en/help/product/wow/197/834/solution
If it goes on cooldown and does not help, then it should open up other options on the stuck page.
A UI reset is also recommended on one character login issues.