Does this happen when you are trying to use cellular data? If so, are you sure you have a reasonably strong signal and have "use cellular data" switched on?
Does this happen when you are trying to use Wi-Fi? If so, are you sure that you are connected to a wireless network and have "use Wi-Fi switched on"?
Does this happen when you are trying to use Wi-Fi? If so, are you sure that you are connected to a wireless network and have "use Wi-Fi switched on"?