Some of my thumbnails in photo's are complete red.

I had the same issue and traced it to either a dying gt120 or incompatibility between NVIDIA and ATI cards in the same machine. Either way, pulling the (unused) gt120 from may Mac Pro fixed the issue. I still have some corrupt thumbnails but those can be fixed by rotating each one with command-r and then back again with option-command-r.

what version of the OS do you have? What has changed since it worked

Generally this is a graphics card issue

LN