I had this same problem on my system with a GTX 980Ti.
What is your monitor setup?
I fixed it by moving both my monitors to DP (2x1440p).
My secondary monitor is dual-link DVI only, but I luckily had a dual-link dvi to DP cable.
If you have more than one monitor and one of them support DP, try disconnecting all other monitors and see if you get signal.
Also if you use iMac15; iMac17 or MacPro6 I guess you have already patched the AppleGraphicsDevicePolicy.kext? (Or renamed your Nvidia to GFX1) If not do that first.
ok, let's take them one by one.
I'm using a dual-monitor setup. One monitor connected via DVI, the other one via HDMI. I will try to disconnect one monitor and see if that makes any difference. I don't have DP on either monitor.
I'm using iMac 14,2 system definition. And by patching AGDP.kext you mean setting those devices to none? Does that even make any difference? I did that in El Capitan. But...I don't think that actually made any difference whatsoever.
How do I rename to GFX1? Is there a way to do it from Clover (kexts to patch section)? That would be really useful.
Well Hooray! It does boot with a single monitor (I kept the DVI one).
Now, how do I get it to work with both monitors?
Well, that's fixed as well. By moving the DVI display to, well, the other DVI port. ) Are you kidding me? THAT fixed it?
Anyway, thanks a lot for the tips and fast reply.