Why doesn't my iPhone 3GS connect to the cellular data network?

I just bought a refurbished iPhone 3GS from Gamestop a few weeks ago and have just started using it recently. I can make/ receive calls and texts with no problem, but for some reason my phone will not connect to the 3G network. I have 3G turned on and the symbol shows up in the top corner of my phone, as if it were working normally. I tried updating my phone, but there was no difference.

It might have something to do with your carrier. Your question should have specified what your carrier is.