my money is on you having a data alert and restriction set up on your phone.
go to settings > data usage and you should see three check boxes. all three or at least the first two are probably checked, and if you scroll down, you'll see that you have hit your self-imposed data cap. you can drag the limits upward and the triangle will go way, and you will be able to use data again. you can also just uncheck the "limit mobile data usage" checkbox and the alert checkbox and you'll be freed again.
that being said, if you're on a data "diet," you will go over if you do this, so check your plan first!