Visitor status on my site with Membership Plugin

I've done something, and don't know what it is. My website has a membership plugin. I'm still tinkering. For some reason, now when someone navigates to the site, who is not logged (ie: visitor) they get an Internal Server Error instead of seeing the site. If you're not logged in - not a member - you can't see the site. Obviously this is not what I want. Suggestions please?