My cellular data won't work for my IPhone 5s (AT&T Carrier)?

I have an IPhone 5s with AT&T as my carrier and recently my cellular data/internet hasn't been working at all. It will show that I have full bars (enough reception to where it should be working) and the LTE or 3G will show up at the top of my screen but when I pull up safari or google it just says "Could not activate cellular data network". I've had this phone for a few months now and it only recently started happening so I'm stuck to having to use Wi-Fi when I can. Rebooting my phone by holding down the on/off and home button hasn't worked and I also checked my Settings>General>About section and I still have plenty of data left on my plan. Would rebooting fix it the problem (or just anything else really)?

Have a word with AT&T. You may have exceeded your data allowance.