rian, it may be a matter of just adjusting the APN settings. I know one of my phones did the same thing and even though I tried to change the settings myself, it wasn't until I called a Straight Talk tech support representative was I able to get an internet connection again. He was able to give me a setting that I wasn't aware of.
First call Straight Talk at 877-430-2355. They can diagnose the problem and even if your phone is out of warranty, they sometimes will be able to help you with an issue like this. Be patient as their support is outsourced and sometimes it is difficult to understand the representative but most are very helpful if you give them a chance. They will walk you through the procedure to change or adjust the APN setting if that is the problem you are having. Even if it isn't, they have the obligation to help you correct the problem if your phone is less than a year old and under warranty.
No, most Walmart stores are not equipped to help you with technical issues. You may just happen to get an employee at the electronics desk who knows more than most and may be willing to help you but the stores generally are not set up to give you in-store support. If your phone is under warranty (you've had it less than one year) Walmart employees are instructed to tell you to CALL Straight Talk as the procedure to have warranty work done is to first call Straight Talk as they administer the one year warranty on all their phones regardless of the manufacturer.