Why is this happening?

This usually happens when

1. You uninstall JUGA *before* unpublishing the JUGA System Plugin or
2. You uninstall JUGA *before* de-activating the JUGA-Enabled Admin Template (for Joomla! 1.0)

Solution

Next time, remember to unpublish the System Plugin first!  But for now, all you need to do is turn off the JUGA System Plugin or de-activate the JUGA-enabled Admin template.  Either way, you need to access your database via phpMyAdmin (or whatever you prefer to use).  Go into your #__plugins / #__mambots table or #__templates_menu table, depending on whether your issue is #1 or #2 respectively. 

For #1: In the #__plugins / #__mambots table, search for the JUGA System Plugin/Mambot and set the `published` field = '0'. 

For #2: In the #__templates_menu table, set the `template` field = 'joomla_admin' where `client_id` = '1'