P2219H, no HDMI signal, two laptops

Hello,

I have just purchased a P2219H monitor with HDMI cable. For the first two days, it worked fine, but today morning I started getting an error message "No HDMI signal from your device" (I did not change the set up from yesterday in any way though). I have two DELL laptops, and I receive the same error message on both. Although when I check Display Settings on my laptop, I do see that two monitors are connected to it. 

I did follow the instructions posted here but it did not work. I tried several times.

I will be grateful for any advice!