The last thing I recall doing was playing with the views and it wasn't until a little while later I noticed that 20,000 files were suddenly unassigned - but it's not a view setting - when I review the tags the details are now missing and I have restored the preset view in customise view.
You may not realize it but you can actually assign fields by drag-dropping files from the listing into one of the Tree items. You mentioned you were playing with the fields. Is it possible that you accidentally may have dropped one of the "children" of one of the Tree items onto some other Tree item and erased the Artist field?
For example, if you do this:
Have 1 View Scheme that contains (for example) [Genre], [Artist], [Album] and is sorted Alphabetically by those categories and has "Populate Tree" checked (so that you can drill down through the tree).
Have another View Scheme that has [Keywords], [People], [Places] and is sorted by Date Imported -d (so that the newest files are at the top) and also has Populate Tree checked. Now say you don't have many keywords assigned, but you do have a few because you downloaded some of the files from Gnutella and some other people assigned the keywords to them (this isn't specific to that tag, just an example). Now, if you accidentally drag the "Unassigned" item from the Tree View (not the files themselves, but the Tree item) and drop it into the first View Scheme on top of "Unassigned" under Artist it will reassign ALL of the files with no keyword to "Unassigned" Artist (which deletes the Tag).
This is just an example, of course, and the "Panes View" also supports Drag-Drop. When I've messed up a bunch of files in one fell swoop, I usually find it's because of an errant Drag-Drop. Of course, MC does ask you if it's okay to reassign them if there's a bunch of files, so it's a bit suspicious.
As far as the files themselves being unaffected, that is an Option you can turn off in MC. Go to: Tools --> Options --> General --> Update tags when file info changes. If unchecked, MC keeps the tag info in it's database but doesn't sync the ID3 tags in the files themselves.