After I have imported and exported photos from a date file, then move onto another date file, I cannot access the photos from a previous date file.
Do I need to close the date file currently open before accessing an older date file ? The photos in the prior date file are dim and cannot be re-imported.
When you say "date file" is sounds like you mean "date folder".
When you do an import, the image files are placed into a folder, usually named for the date taken. They are also referenced in the system collection named "Previous import" at the top of the navigation pane. Each time you do a new import, the contents of the "previous import" is updated with the new import, just as the name implies.
Lower down in the navigation pane on the left is the Folders panel where you will find all the dated folders. If that tree of folders does not look like the tree that Finder or Explorer shows you, right-click on one of the folders and select "show parent". (Repeat as needed.)
Click on All Photographs in the Lightroom library module. It's on the left, under Catalog. Then you can scroll up and down to see your other photos.
Or use the Folder Panel, as explained by ManiacJoe