Portals don't register and do not load at all

After having gone into the nether and trying to go back into a portal, they just stop working. The swirly purple doesn't appear, the loading screen doesn't appear, and you just stand there while nothing happens. You can walk through portals, but nothing changes. It's as if they're displayed but not actually registered by the game. I've tried closing and reopening the world, but it doesn't help.