Many times it happens when you change the template , the virtuemart Add to Cart function does not work .It may happens due to template java script like a template which is using Jquery so due to this Ajax function does not work .So.i’ll suggest that you can use this feature by disabling the Ajax in Virtumart.
Go to Virtuemart Admin->configuration->site->layout->select theme->use ajax to add,update or delete product from the cart to NO. Cheers 😉
