Cannot log into the admin panel

I posted to the community, but realized I may need help here? My site is I logged into my admin panel last night and everything was fine. I've spoken to all my admins, and no one made any changes other than permissions to new members. This morning, when any of us try to log into the admin section (multiple admin accounts from multiple locations and browsers), we all get the 'processing bar at the top right and get kicked back to the log-in screen without getting in.

Things I've tried:
  1. clearing browser cache
  2. multiple browsers
  3. different computer
  4. run TRUNCATE xf_session_admin

I can contact my sever host, but don't know what to tell them exactly. I saw a post from a couple years ago where someone resolved this with flushing memcache, but I'm not sure how to do that. I just invited a ton of new members to the site who cannot get access to the forums, so panicking, and no idea what to do. Any help would be greatly appreciated!