Niantic has confirms reports of loading issues in Pokémon GO and has provided users with a temporary fix. Read on below to learn more:
Trainers, we’re investigating reports that Pokémon GO is experiencing issues loading for some users. We apologize for the inconvenience, and will update here when we have more information. In the meantime, uninstalling and reinstalling Pokémon GO should let you access the app as usual. We are continuing to look into the root cause of this issue, and appreciate your patience.
Source: Niantic Support @NianticHelp