Well all we know is that it's "stopped working", which isn't a lot to go on. So all I can do is give a few checks and tests that I'd try myself.
Firstly, you say "4G", so I assume 2G and 3G are working for you, or have you lost data or network contact completely? Is the problem restricted to a particular location (i.e problem with a particular cell)? You've tried rebooting the phone, and removing and replacing your SIM? If you have access to another handset, have you tried the SIM in that? If 4G works in another handset then it's your phone, if it doesn't then it's your SIM, your account or your network (networks can have outages that affect some users but not others). Have you tried calling your network operator to see whether they are aware of any problem?