You disable Intel Graphic Card didn't you? You did right? And I'm here saying you can't disable Intel GPU because Nvidia GPU needed it to operate normally. Also, normal BIOS will not let you disable Intel GPU because of that reason.
If you still can access BIOS try re-enable the Intel GPU and that should fixed the problem. But I doubt you can because Intel GPU is disable... aka. nothing control the graphic output anymore.
PS. I think you tried it already but I'll say it anyway. If normal power button not working try Lenovo button next to it should have some effect. If not... remove the CMOS battery and wait about 5 mins should clear the setting... If not... yeah... replace the CMOS chip is your best choice...
Edit : More info on your problem.
https://forums.lenovo.com/t5/Lenovo-P-Y-and-Z-series/Y50-disable-integrated-graphics-card/td-p/3349856