No ‘hdmi signal’ on second monitor potential fix:

To me, that suggest that cable is marginal.

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

corinna296 wrote:

I see other people have had the same problem as me with a Dell monitor on their MacBook Pro showing ‘no hdmi signal’. Mine was working fine and then suddenly not. Someone on here said something about it being a feed back issue, so having tried restarting comp, holding down option key in settings, unplugging everything and plugging it back in and finally trying downloading the latest operating system ; no joy.

I then tried just plugging the hdmi cable in the other way round . And it worked!

hope that helps someone else.

Good computing corinna296 !