$\because$四边形$ABCD$是平行四边形，$\angle A=90^{\circ}$，
$\therefore$四边形$ABCD$是矩形，
$\therefore S_{四边形ABCD}=AB\times AD=7cm\times 6cm=42cm^{2}$，
故答案为：$42cm^{2}$.

$\because$四边形$ABCD$是平行四边形，$\angle A=90^{\circ}$，
$\therefore$四边形$ABCD$是矩形，
$\therefore S_{四边形ABCD}=AB\times AD=7cm\times 6cm=42cm^{2}$，
故答案为：$42cm^{2}$.