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 iPhone? 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.