Calendar shows an !

I have an iphone 6 and I always used to have my Facebook friends’ birthdays showing in my calendar app. (in a blue colour). I noticed the other day that these do not show anymore, and that in the app there is an exclamation mark (!) in front of ‘calendars’. I have tried just about everything and have no idea how come my iphone’s calendar doesn’t sync with my friends’ birthdays in Facebook and I have no clue as to what the ! stands for and how to solve this issue. Please HELP!