The plugin -does not- have any functionality in it to mass delete reviews, not automatically or manually. There must be some other plugin that is interfering with it.
Since all of the reviews are missing from admin, it rules out a theme issue, since themes usually only control things on the user side of things.
Since the page is coming up 404, it means that something is completely keeping the reviews page from even being shown. WordPress itself controls 404 pages, so this would have nothing to do with the plugin as far as the 404 page.
I would suggest listing all of the plugins or customizations being used on your WordPress installation (and the version of WP you are running). You could also try disabling each one-by-one and then clearing any caching plugin you may have installed.
We haven't received any other reports of this happening from other users of the plugin (over 7,000 now) , so I'm guessing it is localized to your WP installation somehow.
We'll get it figured out!
If you want me to take a look at it, you can email me some info for your blog to aaron [^at^] gowebsolutions.com and I will see what I can find out. If you have access to cPanel on your server, that may also be helpful login info to have so I/you can check the raw database for the plugin and see if the reviews did indeed get deleted from the database somehow or if they are still there.