My connected dell S2415H display is no longer workingg

I have a Mac Book Air purchased 2 mos ago. I am connected to a Dell S2415H 24" monitor. worked fine until a little over a week ago. Now I get a message that my computer is not sending a signal to the monitor. I think this started after a recent update to OS Sierra. I've tried rebooting both, checked connectors, even replaced HDMI cable but nothing works, the display still doesnt sync with my Mac book. Any suggestions? All updates have been installed.