Why does the computer not see the monitor?

Question

LG

I repaired the computer, turn it on - there is no signal on the monitor. Installed another one, same thing. I decided to take the computer home for diagnostics - everything works fine. The VGA cable and power supply were the same for 3 monitors during the test. No signal is written by ACER monitors of different brands, but LG shows. Actually, what's the problem? Tried to change VGA, power - did not help. one of ACER'ov writes "the cable is not inserted" while it is not inserted, and if it is inserted, then immediately "there is no signal". Here is such an anomaly.