Why doesn't my Samsung Galaxy A5 connect to my mobile data?

[AC Question]

I recently upgraded my contract with O2 and the mobile data had been functioning fine. However, today it doesn't work whenever I try to put it on even when Wi-Fi is disconnected. Since I have just renewed the contract I have a sufficient amount of data left but this is frustrating as I need mobile data for my everyday life.

 

[belodion]

Try first a Power Off and Restart.
If no go, seek advice from O2.