No Colour in Thumbnails

[John Morris]

I use graphics files to a large degree and rely on
thumbnails to access my pictures. For some totally unknown
reason most of the thumbnails of graphics on my computer
have lost their colour and have turned into shades og grey
and black (no white!) If I right-click on an individual
thumbnail and select "Refresh Thumbnail" the colour comes
back into that one thumbnail but no others. Could someone
please tell me :
(a) Is there an overall command somewhere
which will restore all the thumbnails, and also
(b) Has a anyone any idea as to how this
could have happened ?

Many thanks,
John Morris