My iPhone is not letting me use Internet?

I have an iPhone 5. And recently whenever I don't have Wi-Fi my iMessage doesn't work and my Internet doesn't work. Like the apps I have such as Instagram and twitter don't load because it says I don't have an Internet connection and my safari doesn't load either because of the same reason. It's worked fine since I got it back in December but now it has all of a sudden done this so I was wondering if there's any particular reason it's doing this or if my phone is messed up and how I can fix it.

Your data is turned off. Go to your network settings and turn data back on (3g)