This is a trick error with very little information floating around on it. There are various reasons why it happens but essentially something has caused your default store view to forget what the default category is.

magento may throw up a

Mage::throwException(’Invalid categor…’)

error in the diagnostic info.

The fix is easy. Log into admin which will still be working and under configure select manage shops and click on your shop name.  Your root category will be empty .. simply select the correct root category.