Iphone 4 3G wont work.

I have sprint and I recently got an iPhone4 and Ive had it for about 2 months. For about the past week it wont connect to 3G like it used to. I used to be able to get on the internet whenever on my iPhone even if I wasn't at home because of my 3G. But its not working no more. it has the little 3G symbol up and it puts the loading sign up but it never loads. It wont let me connect to the internet unless I have wifi and this shouldn't happen I shouldn't need wifi. I only use the wifi when i am at home and I Dont need my 3G but its not working anymore. I have tried everything. I even tried resetting the network connection and all that Ive shut it off and on. I don't know what else to do. Help please?