I had to buy a new monitor to connect to my lap MacBook Pro running OS X El Capitan 10.11.6 (makes it easier to see at my desk) just like I had in the past. I connected the HDMI cable to the LG monitor and I got nothing. The monitor screen says No Signal. I have tried everything. My toshiba is all I had to do is connect the HDMI cable. What am I missing?