After upgrade 1.7 to 1.8, ‘Place Order’ button does not work

Replace the base file the following path 

app/design/frontend/default/your_theme/template/checkout/onepage/review/info.phtml 
(or) 
Replace the line number 81 in info.phtml 
review = new Review(’<?php echo $this->getUrl('checkout/onepage/saveOrder', array('form_key' => Mage::getSingleton('core/session')->getFormKey())) ?>’, ‘<?php echo $this->getUrl('checkout/onepage/success') ?>’, $(’checkout-agreements’));