Do both cameras not work, or just one of them and, if so, which is the one that doesn't work?
A quick - fixes many problems - solution is to reboot the iPad by pressing and holding the Power and Home buttons for about 10 seconds until the white Apple logo appears; the iPad will then reboot.
See if that fixes the problem but, if not, get back to us with some more information and we'll try and suggest some more possibilities.
Tim