Problems stacking Arduino Shields
Make sure you are using a software serial port for the GPRS shield. 

If you aren't using the software serial port, then you are probably running into an issue with trying to read data over the only UART (serial) port while two devices are tryiing to use it.  

It looks like there are two jumpers next to D0/D1 on the GPRS shield, I can't tell but I think if you remove those, the shield will use software serial over D7/D8.  If you do this, you'll have to code your program to use a software serial.  This wiki page on the GPRS shield if you need help with that.