Monitor wont get signal when computer turned on

Hello,
my sister's computer didn't turn on so I decided to switch the PSU from my computer to hers in order to confirm it was a PSU problem.

indeed it was since her computer worked, but when I returned my psu and connected back all my system components, the computer turned on, heard a beep as usual, however the monitor remained in sleep and even after I turned it on it said no signal and turned to sleep again.

I checked out all the connection and maybe I missed something. I just cant find out what, any solutions to the problem?
(it may has nothing to do with the psu, but it seems connected since it all worked perfectly before today)
 
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