Probably the easiest way is to reboot the iPad; press and hold the Power and Home buttons at the same time until the white Apple logo appears - it can take as long as 20 seconds - and the iPad will reboot. Should fix this.
Another possible reason is that you've accidentally turned 'Restrictions' on in the 'Settings' menu. In 'Restrictions' there is the facility to disable Safari, so check that's not been enabled. It removes the icon from the Home screen too.
Tim