Suddenly locked out of Win11Pro desktop

I run a Media Server on a Win11 desktop PC. I connect to that through WiFi in my TV room with an HP AIO that is connected to the TV using a HDMI cable. I do this to stream videos from the Media Server to the TV using the HP AIO.

Last night, while watching a video, the screen suddenly went blank. This is a daily occurrence due to Verizon dropping their signal on a regular basis. When this happens, the network icon on the AIO changes and we wait for it to change back. Then, I reconnect to the Media Server and restart the video where it left off. A few minutes of aggravation, but something we have learned to live with.

But last night, the WiFi icon had not changed on the HP AIO and I was able to access the Internet on it. So, it was not the usual networking issue.

I went to the Media Server to see if it had rebooted (as it does, from time to time), but it was still up and running -- so I tried to access it from another PC -- and got the error message that the account was locked out!

So, I rebooted the Media Server PC and when I tried to login, I also got the error message that the account was locked.

So, I got onto my primary desktop, accessed the router, brought up the network map and looked through the devices listed. I have a spreadsheet of all my devices along with their Mac addresses -- and everything on the map matched my devices. So, there were no "foreign" devices on the network -- and the guest option is locked out. So, as far as I could tell, I had no intruders in my local network.

This happened before a couple of weeks ago, and I "solved" it this time the same way I did last time -- by restoring the PC from an image backup I made a couple of days back. That allowed me to login again and allowed other PCs to access the Media Server again.

I looked this up online but didn't find anything usefull to determine HOW this could be done. My guess was that someone had accessed my local network through WiFi and somehow, changed the login for the Media Server. But I did not see any evidence of any other devices, so I have no record of anyone doing that.

I could change the router to populate a Mac address table to only allow those devices, but I'm looking for ideas on other ways to prevent this from happening.