Some additional info could help to understand what happened. For example:
What did you do before the cover art images disappeared? Did MC crash?
What are your cover art options in Options > File Location?
Are the images missing also in the Cover Art visualization in Playing Now or only in thumbnail views?
What does the "Image File" field show? (You can add this column to a details view.)
Have you tried to do update tags from files? (Check that the "store in file tags" cover art option is enabled and try this with some separate test files.)
Have used the "update for external changes" Auto-Importer option?
What is your OS and MC version?
You mentioned that you have used WMP besides MC. WMP may do unwanted things without asking permission. For example, if it happens to download file data it can replace an existing higher resolution folder.jpg with the "stamp-sized" MS provided cover art. I have no idea how it handles embedded cover art in WMA files.
IMHO, the safest and most convenient MC option is to use external "artist - album.jpg" besides the album files or in a separate folder. Then WMA does not alter MC's cover art files.
I don't remember anything particular happening before I noticed the covert art missing problem. It is possible MC crashed, but I really don't remember for sure.
My cover art options are: "In the same folder as the file (as Folder.jpg)" and "Also store image in file's tag" is checked.
The images are missing in Covert Art visualization, but thumbnails are fine.
When I view the Image File field it is empty. Also, if I right click on a track and go to the Cover Art menu, the "Copy to Clipboard" option is greyed out, so MC definitely thinks there isn't covert art in there.
I just tried "update tags from library" and it said it updated 1 file with 0 errors (I only tried 1 file as a test), but the "Image File" field didn't change.
I have't used the "update for external changes" option in a while, but I can't be sure that this problem didn't begin when I last ran it. I just tested re-importing a single folder via Windows Explorer's Media Center context menu and that
did seem to fix the problem. I then tried an auto import from within MC and that had no effect. So, then I tried the context menu import on my whole library and that seemed to fix everything. I don't know why the auto import and the context menu one don't work the same way.
I'm running XP SP2 with MC 12.0.240, although I upgrade reasonably frequently, so the problem almost certainly happened initially with an earlier version. I'm guessing about 2 or 3 weeks ago.
If I notice the problem happen again I'll try to pay attention to what I did before that might cause it.
As for WMP doing strange things, I realize this is a possibility, but I believe I've set it not to. Besides, I don't generally use it, only did this time as a test outside of MC. Why would I use WMP when I have MC.
Thanks.