This same thing happened to me few weeks ago...I was checking the server load, it was 0 0 0. So, I login to my site, I saw the message "Licence Key Invalid" Apparently, my site went down for 2 hours already.
After I clicked the message, it led me into ACP, I didn't fix anything, the message was gone, and my site went normal.
Do I need to worry about 3rd party app? 🤕