Check the signal cable?

I was searching too much on the internet, without finding solutions, I initially thought my problem was screwed, and that the screen and/or the graphics card had a defect, until I read your response, misterden, thanks ;)

Thank you, but I don't have access to any buttons on the side. When I press one of them, it shows digital, analog, and then returns to blinking (sleep mode)!

Thank you Mitterrand, you saved my life.

Thank you very much,
I had the same problem and your answer saved me!!!