Update: According to Box's status page, it looks like the issue has been resolved. Carry on!
Are you amongst the many using Box for additional cloud storage on your Windows Phone device? If so, have a look at your account and see if it's logged in right now. Many have noticed their accounts have automatically logged them out and when attempting to log back in, the 'Grant Access' button no longer works, leaving them unable to log into their accounts. Box has now updated their status dashboard indicating they are aware of the issue and are working on it.
Degraded Performance: Box Notes, Box Sync, Content API Subscribe - Investigating - Users that have logged out of Box Notes and Box Sync or are attempting to use the content API to authenticate may not be able to log in.
So, that's that. If you're having issues with Box, you'll just have to wait until they clear things up. We'll keep an eye on it and post an update when they change their status page to show everything is well again.