Twice I’ve had the Theme Switcher plugin (v0.2) activated and switched to a Theme where I didn’t have the theme switcher code in the sidebar and I got ‘stuck’ in that theme regardless if I deactivated the plugin or changed the Active Theme in the Dashboard.
So I deactivated the plugin, deleted the wp-content/themes/xxxxx directory (xxxxx being the offending Theme), changed the Active Theme to one that had the correct switcher code in the sidebar, activated the plugin, and all was good.
Of course I reinstalled the deleted Theme and added the theme switcher code to the sidebar.