It is very possible that whatever tool you used to do the import ended up causing some corruption in the database.
The first thing I would suggest doing is creating a new custom menu and assigning that as your site menu. If that works, you can just recreate your menu structure.
You can also force WP to run the built in database repair process (Tutorial
), and if that doesn't work, there are a handful of database repair and optimization plugins you can try (just search for database repair on the plugin install screen).
Unfortunately, there is a possibility that the Joomla import caused permanent damage to the database, but your best option is probably assigning a new site menu.