Are you trying to download using 3G or WiFi. Remember that 3G downloads are limited to 20 MB. However, do a 'power-off, power-on' reset of the iPad. Hold the Power button down until the slider control appears then accept the offer to switch off completely. When the screen has gone completely blank, press and hold the Power switch again until the white Apple logo appears. The iPad will power up; it takes a few seconds, so be patient. See if the problem persists but get back to us if there's still a problem...
Tim