Unusual desktop appearance

[m654321]

Christmas Eve: I went to turn on laptop (1) - see signature - then selected LL from the grubscreen.

Everything booted as usual, and left the laptop downloading TV programmes using get_iplayer. When I returned to the laptop later to check on things, I got the shock of my life ...

- the LL manual icon on the display was changed into a red triangle, with "danger" written on it
- my other Desktop icons were white-ed out, and had a red cross in the middle of all of them.
- icons on the bottom right panel (WiFi, clock, volume control, etc) were missing and when I moved the mouse around
  a few more disappeared
- touchpad or mouse clicks produced no response to either the menu button or any other Desktop icons

Therefore, I had to resort to the power-off button to shutdown. Then the following happened the next day ...

Christmas Day: returned to laptop (1), power on, and all I got was a black screen with a blinking short white line, near the top left hand corner of the display. >

The only thing that has changed with our home network, over the last few days, is that my elder son & girlfriend are staying with us over the Christmas period and have brought their laptops with them, and of course they are running off our router for the internet connection. They don't have Linux - they both have Windows 10.  Is it possible that they have inadvertently introduced something malicious to our home network, particularly if they haven't taken the security measure of having dual account set-ups on Windows 10, i.e. an admin a/c and a daily user a/c? I'm suspicious ...