My Reminders won't sync between my devices anymore!

I noticed yesterday that my Reminders and my Calendar won't sync between my devices anymore (iPhone, iPad and MacBook). I've just spent over an hour online trying to find a solution, but nothing is working. I've shut down and reopened the app. I've shut down and reopened my iPad and iPhone. The Reminders and Calendar are both enabled in iCloud. And it working up until yesterday!