site down - "Module XML data not available"

re-installed joomla and hikashop. Hikashop modules don't work which means that customers can't buy, which means that the owner of the site is very disturbed. On the module it says "Module XML data not available"

It sounds like you updated joomla from a 1.5 website to a newer version but didn't reinstall HikaShop, or you did but you have access right issues in your modules folder which prevented joomla to update the XML files. Because of that Joomla cannot find the XML files as the structure for them is different since 1.6:
docs.joomla.org/What's_new_in_Joomla_1.6#XML_Manifest_Changes
The simplest is to edit the xml files in modules/mod_hikashop, modules/mod_hikashop_cart, etc with a text editor via FTP and change <install and </install to <extension and </extension and joomla should then be able to find them.