See the following for reference.
KB02334-Network status indicators from the GSM/GPRS/EDGE/3G/4G networks
Now, assuming you are not experiencing a data outage and your account with your service provider is ok and includes a BlackBerry data plan, see the following.
KB01631-BlackBerry smartphone network indicator is GSM instead of GPRS/EDGE Posted via BlackBerryForums.com Mobile