Magento - 下订单进入购物车页面但订单显示

I am running Magento 1.8.1.0 on my own VPS with 1and1. Everything was working great but now all of a sudden the checkout no longer works. Instead when you click "Place Order" it goes back to the cart page, the order is actually placed and shows up in the backend. No emails are sent or anything, and products stay in the cart still. I am running Magento 1.8.1.0 on my own VPS with