you're lucky - I kept a version 1.0 zip archive of the plugin: OnlineVisitor-1.0.zip
Find the addon folder in /data/_addoncode/[123abc] where 123abc are some some random characters - and replace the files with the ones from the zip.
You sholud see version 1.0 running again. Hope it works.
Is there any way of reinstalling?
You can uninstall the plugin and extract the content of the zip archive into your /addons directory, so you have /addons/Online Visitor/[addon files here]
The Plugin will then be available for re-installation in the Available Plugins list on the Manage Plugins Admin page.
Hope this helps.