I was moving a Joomla! 1.5 site to a new hosting server. The site had been running on PHP 5.2 on the prevoius server but on the new host only PHP 5.6 or later was available. Joomla! was version 1.5.14.
After installing the site on the new server, no menu items were visible, not in backend, nor in frontend. I checked the database tables and the menu items were present in the database, they just didn’t show up in Joomla!.
The solution was simply to upgrade Joomla! to the latest verion, 1.5.26, which is compatible with PHP 5.6 (something that I would have done anyway as the next step).