Magento – Fix Customers Being Redirected After Pressing Place Order

Having a major issue on my live store!

When the customer goes through the one page checkout, all is fine, but when the customer has got to the last step where it says place order, this should redirect to the success page (Using card payment).

Instead what happens is the customer gets redirected to the cart page, the items remain in their cart and they have no emails or order confirmation or anything! Also in the backend of the site, we have no order information coming through at all, so we cant see that this customer is trying to order on the site. The only reason why i found this problem out is because, although the order wasn't placed on the website, the money still gets transferred into my paypal, so the payment is being taken but we have no idea its happening except the emails that are coming through from paypal.

What do i do!?! Anyone know why this is happening?

To clarify, no order information is being stored on the backend or sent to the customer, and they are just being redirected back to the cart.

Thanks

EDIT

I have just put my site back into sandbox mode for paypal who deals with all transactions on our site, and i processed both a paypal order and card payment order using the sandbox and both orders went through perfectly, so is seems as though the issue is occurring when i set my payment methods to use paypal live…?

I checked var/log/exception.log and i have no errors which are related to the issue.