A bit of a stab in the dark, John (you have not said which Windows or which OE). Unless the email has loaded a virus (do you have AV running?), I would go for a corrupted email store folder.
Do you know how to copy the email folder? I would advise doing that first to cover any problems, OR do a back-up of your 'documents and settings' folder. PM if you are not sure.
ONLY THEN, in OE, click on 'File/Folder/Compact all folders'. Could take a while!