I have had this problem with my yoga-730 after I replaced the keyboard and other things. Each time it was something different so it's just a lot of troubleshooting. You may have to unplug the display connector then plug it back in. One time I took out my RAM stick and it started perfectly I don't know why that worked but that happens a lot lol. Try different things and they may work you never know. It is also good to read the manual made by Lenovo, this can open your eyes to new things you never knew about your laptop.
https://download.lenovo.com/consumer/mobiles_pub/lenovo_yoga_2_pro_hmm.pdf
It's right here^^
Let me know how everything goes. Good luck!