Grey thumbnails usually are an indicator that something with your monitor profile is wrong.
Try to recalibrate the monitor or for testing purpose switch to the sRGB profile and see if this helps.
Anyway, and if it does not help:
There are many confirmed issues with LR 6.2 - you may try the steps here: Update on Lightroom 2015.2 / Lightroom 6.2 Release
Anyhow, you are best helped if you roll back to 6.1.1 as many users: http://www.lightroomqueen.com/how-do-i-roll-back-to-lightroom-2015-1-1-or-lightroom-6-1-1/