The folders are under a master folder, and another sub-folder (at the same level) has remained. I have already processed the files and exported JPGs, which also used to be there but now are not. I have not moved anything or had a crash. I am on a WinXP machine and can see all the sub-folders and images in Windows Explorer. If I go back to LR to re-import them, they show up but are grayed out, so cannot be re-imported. Why would this happen and what can I do? I need to export some of these for client use.
I don't know what exactly caused this, but your photos were imported into Lightroom and remain imported into Lightroom (because you can't import them), just not in the folder where you think they should be. (My guess is that you made a mistake on import)
So, you need to find one of the photos in Lightroom (search by file name in the filter bar), and then locate which folder it is now in by right-clicking on the photo and select Show Folder in Library.
Now you know where this photo is located (and presumably all the other photos in that folder are the ones that are supposed to be in the same folder as the one you found), you can either leave them there, or update the folder location back to the original location (see Adobe Lightroom - Find moved or missing files and folders, scroll down to Updating Folder Location)