Problem with Dell screen "power saver"

Hello,

I have taken a Dell monitor (e2211h) that I tried to connect to my HP tower, but every time I start it by pressing the on/off button, a message appears on the screen saying "Entering power-saving mode" followed by a logo of a VGA connector.

I have changed the cables and checked if they worked on another computer (and all these tests were conclusive). If I press one of the 3 buttons at the top, another message appears saying "No signal is being emitted by your computer..."

How can I fix this issue? Because I need it quite urgently.

Best regards,

gqdThinky