Does your Mystic Vision portal have a Red X? Is the portal still counting down? Have you tried quit and restarting your app? Or even restarting your device? Try getting on a Wi-Fi you know that works, reset your in-home Wi-Fi router to see if helps. Does updating the game to the newest version/build help?
A full day of not being able to access the portal seemed quite long and odd. If you have tried everything and still unable to view, you may want to send support a ticket from in-game.
