This just happened to myself and 2 other people reported the same problem.
If you own an Nvidia Graphics card, do not update to Driver version 378.66 !
Installing this version will cause Preview Window in Vegas Pro to go black and no videos on timeline will show.
If this has happened to you, roll back to an older driver version.
Open Device Manager (right-click Windows Start in Windows 10) or select from Control Panel.
Open Display Adaptors, select your Graphics Card then right-click and select Properties.
Go to Driver tab and press Roll back, if it is available.
If it is greyed out, there is no previous driver version saved in your computer
If no previous version is there, you can do a manual search for your Graphics Card Driver and set the ALL.
This will show all driver versions every released - you can download and install an older version from here.