Just yesterday the popup window after clicking 'add to cart' would show up however just last night it stopped working. I don't think I changed any settings relating to AJAX so I'm not sure what's going on here. I have installed a module called 'Virtuemart Products Slider' however the demo for the module show its working with the popup. You can see my site here http://twp.gsqtest1.com/
Something is stopping the popup. Looks like you are loading both fancybox and facebox javascripts - you could try changing the "Use Fancybox" setting in VM configuration/templates. If it worked before adding your slider module then try with it disabled A demo is not a good example of function unless the demo uses exactly the same template and additional scripts as you do.
You have a coding error showing in IE10's debugger. On the cart page in the following line snippet onsubmit="check(1this) is wrong - should be onsubmit="check1(this)
<input type="text" onblur="check1(this);" onclick="check1(this);" onchange="check1(this);" onsubmit="check(1this);" title="Update Quantity In Cart" .........
Check your template override. The code block where you have an error (in VM2.0.24) is:
onblur="check<?php echo $step?>(this);"
onclick="check<?php echo $step?>(this);"
onchange="check<?php echo $step?>(this);"
onsubmit="check<?php echo $step?>(this);"