Find the location of the mail boxes via OE, Tools, Options, Maintenance Tab and click Store Folder. Close OE.
Using explorer navigate to it and move the inbox(1).dbx to a different location. Go to the new location and rename it to inbox.dbx.
Now simply copy the correct inbox to the Store Folder location. If should ask if you want to overwrite it, say yes.
Open OE and it should be back to normal.