I am not sure what version of x-cart you are running but here is the one on mine. (3.5.7)
Code:
{if $product.forsale ne "B"}
{include file="buttons/add_to_cart.tpl" style="button"}
{if $login ne "" and $active_modules.Wishlist ne ""}
{include file="customer/add2wl.tpl"}
{/if}
{else}
{$lng.txt_pconf_product_is_bundled}
{/if}
{else}
{include file="submit_wo_js.tpl" value=$lng.lbl_add_to_cart}
{/if}
{/if}
Look in your skin1/customer/main/product.tpl
And see if you can find lines similar to those and see if you are missing anything.
If not you may have accidently deleted an image or skin1/buttons/add_to_cart.tpl