Checkout Button Suddenly Broken

[stephen1215]

Hello. I would really appreciate any help fixing this problem.

When my users try to checkout using the "Checkout" button, it take them to a page that gives the following message:

404 Not Found

The requested URL /store/index.html" target="_top was not found on this server.

The url looks like this:
http://www.mysite.com/store/index.html% ... et=%22_top

I'm losing many orders because of this problem, and I can't figure out where to check that the "Checkout" button link is linking to the correct place.

Thank you for any help that you can offer to help me solve this problem. I'm using ShopSite 4.