Woocommerce adds the action hook at priority order 30.
add_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart',30);
So, You may have done
remove_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart',30);
to remove the action from the hook. To remove an action the priority must match the priority with with the function was originally added.
Now you can do
function add_cart_button(){
global $product;
$id = $product->get_id();
if($id != 1345 ):
add_action('woocommerce_single_product_summary','woocommerce_template_single_add_to_cart',30); // You should note this should trigger later tested at 20, 30, 35 working
endif;
}
add_action('woocommerce_single_product_summary','add_cart_button'); // This triggers at 10 by default
To add the button again.
You should check the order of execution of functions in a hook.