Go to Edit > Preferences > Performance, and disable the GPU.
If you are unable to do this, follow this procedure:
Go to C:\Users\Your ursername\AppData\Roaming\Adobe\Lightroom\Preferences (this is for Windows, Mac will be something similar)
There will be a file called Lightroom 6 Preferences.agprefs, open this file in a text editor.
Very close to the end there is a line that says useAutoBahn = true,
Change this to useAutoBahn = false,
Save and close the file, now try to start Lightroom.