Magento / Apache Problem

We have a Magento website that has since stopped adding things to the cart since apache was upgraded, we are stuck in a position where we are unable to downgrade apache nor can we upgrade this version of magento as it has been highly customised. This is affecting a live website from taking orders as when you try to add something to the basket it says 'Can not add item to shopping cart' This needs fixing today in the next coupld hours really.