Seems like the Previews might have got corrupted.
Would suggest you to Quit Lightroom and go to the location where your catalog is stored > Rename Previews.ldata file to Previews-old.lrdata > launch Lightroom and then, check.
Try resetting the preferences - Resetting the lightroom preferences file - updated - Lightroom Forums
If that doesn't help, try setting the monitor profile to sRGB - How do I change my monitor profile to check whether it’s corrupted?
Are you on Mac or Windows? What version?
Thanks Chetna. How do I find where my catalog is stored?
Go to Catalog settings in Lightroom > General > location > which will tell you the catalog location > click on Show > then Quit Lightroom and rename Previews.lrdata folder
And if that does not help, you might also want to try the steps mentioned by Per.
I'm using a PC, so I don't have Lightroom>General. But, in the Lightroom file containing my Catalog, I found a folder named "Lightroom 5 Catalog Preview.Irdata". That folder contains two files (named "previews" and "root-pixels") and 16 folders (named "0" to "9" and "A" to "F").
Should I retitle the Lightroom 5 Catalog Preview.Irdata folder? Or am I looking for a file instead of a folder?
yes, you need to rename that complete Lightroom 5 Catalog Preview.lrdata folder (make sure Lightroom is closed while doing that)
Thank! I reset the monitor profile, and the problem vanished! Thanks to both you and Chetna. Very grateful for the support!
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Since setting the monitor profile to sRGB fixed the problem, it means that your monitor profile was defective.
Using sRGB works because most monitors have a gamut close to the sRGB gamut.
How accurate it will be depends on the characteristics of your monitor, so to get an accurate profile, you should ideally calibrate your monitor with a hardware calibrator.