In technical support, the first and foremost steps include restarting your phone (soft reset) and if possible, remove your battery and put it back in (hard reset). If these steps do not get your data working again, then the following two options might help you.
1.1 Bring up your settings and make sure you're on the Networks tab
1.2 Select Mobile Networks
1.3 Select Mobile Data
1.4 Make sure Mobile data is selected to ON
If this does not get the data working, go back to the Networks tab and select Data Manager
2.1 Raise the warning alert limit by tapping and holding the red line. Raise it up to an absurd number like 67 gigabytes. Also toggle the mobile data tab to off and back on.
One of these two steps should solve your issue!
If not, there's always going to the General tab, down to Backup & Reset and Network Settings Reset and or Settings and Mobile Networks and Access Point Names and hitting the context menu and selecting Restore default settings.
