Display problem....

I am still new to this but have been running OS X for a year or so with several updates being successfully installed. Now I have an annoying issue that I know is a stupid mistake, but I have not been able to straighten it out.
Everything boots up fine and all systems are a go. Then I turn off the display, or switch to a different input and when I switch back to OS X Computer I get a no signal message! The computer is still running and it doesn't seem to matter if it's one second or one day! I have to reboot the computer to get the display to connect again. This has only started with the Yosemite update. Any ideas?

Gigabyte z87mx-d3h
i7-4770K
internal graphics
monitor is a lg led tv hdmi

You may find that switching the monitor off and then back on a few times will work. I sometimes get a snowy picture when the screen saver has been on for a while. Switching the monitor off and then back on again three times is normally enough to sort the issue for me.