I have Woocommerce 1.6.1 and then updated to 126.96.36.199 and then my Add to cart function was lost on the product page. Before I updated it, I backed up my plug-ins. So I renamedthe plugins folder that I updated and restored the previous plugins folder... and the Add to cart function is still not there.
This is the only thing I did and I can't figure out how to get it back. :-/