Plugin keeps disappearing from my dashboard

This is the second time it happened to me. One day it's there and it works then the next day it's not there anymore. It doesn't even show up in the screen options.

It was visible on a tab that hadn't been refreshed for awhile so I clicked on it there and got a message "You don't have sufficient permissions". That's another issue I have, for another post.

I really like having that tab under the dashboard. Any idea why this is happening and how I can make it stop?

David