Thanks, we will try to redirect all broken links to the new pages. It will be a daunting task because all of the broken links were dynamic and we don't know what products or categories its belong to. Since we moved from Able Commerce to Product Cart we also changed our hosting provider also; therefore all files have been deleted from the server.  

Although it can be somewhat of a nightmare to implement, the best strategy is actually to recreate those URLs and make them "301 Permanent Redirects" to the new pages. You can start with some of your main products and categories.

301 Permament Redirects are the "search engine optimized" way to tell a search engine that the page permanently moved to a new location. In Google terms, this means that the PageRank of the old page should be transferred to the new one.

This strategy also provides another important result: any links that point to those products and categories will not end up in a "404 Page Not Found Error", but rather will be redirected to the correct page.

Of course, here we are referring to links that you have no control on. Some of those links might have been "powerful" back-links that were giving significant PageRank to your store pages. If those links now return a 404 error, they will lose their effectiveness, and you definitely don't want that to happen. You need all of the back-links to your Web site to remain relevant. As you probably know, you can query Goole for a list of back-links and see which pages they point to: those are the first pages that you need to address.

This article in the Google Webmaster Center covers your scenario and talks about 301 Redirects.

We hope this helps and thanks for the kind words about ProductCart!

We were in the top 12 on Google on some of our most relevant keyword phrases until we switched over to Product Carts.  Now all product and category links from our old Able Commerce have been broken.  I wish  that I have used Product Cart in the first place since it is far superior over Able Commerce.  How can I fix this problem? Sales have dropped over 50%. How can I tell Google to delete all links that are no longer existed.