Site down after clicking a category

We have had an operationl site at http://www.irishshopper.com with version 1.3.9b but something has happened the site in the past 24 hours.

Now when you go to home page it looks fine but if you click on any of the product pictures to go to that category the page cannot be found.

The obvious question is; "What changes that you know of, were made in the last 24hrs.?"

Other than that, suggest you check in Admin to make sure Categories are all still there and then turn off your URL rewriting module; see if things work normally. Could also try changing to the Classic Template to see if problem still there.

categories all still there.

what do you mean url rewriting

When I mouseover a link I see this URL:
http://www.irishisbest.com/index.php?main_page=index&cPath=91

But after clicking the link I get this URL:
http://www.irishisbest.com/tracking-insurance-p-621.html

BTW, your original problems seems to be gone?