Question No signal from both Computers that plugged in by hdmi and vga

Apr 17, 2021
I have two computers with one monitor that has a automatic switch to display two different sources and it has been working great till yesterday. But this morning I put my first system (plugged in through VGA) to stand by and turn on the other system (plugged in to the same monitor through hdmi) and since then there is no signal from both systems. To make sure if the cause was my monitor I plugged in my laptop'a hdmi cable to the monitor and it worked perfectly fine. I thought perhaps the problem was that both system's graphics drivers somehow got corrupted because of the switch system??so I plugged out the graphic card of first system and restart the system from vga cable(without graphic card driver installed )and this time it worked.i am gonna do the same for the second system too to switch from graphic card driver to built in video card to have a new driver..but what do you guys think as the cause of this problem I am really wondering how a monitor can cause such problem?.. and thank you in advance.
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Update your post to include full system hardware specs and OS information for both computers.

Make and model monitor?

Make and model automatic switch?

On both computers look in Reliability History - Reliability History may have captured some related error or warning event.