Reopen your last LR 5 catalog in LR 5 not the LR 6 catalog.
Tell us what error condition or message you receive when trying to access any of your photos in LR 5.
The first time you launched LR6 it made a copy of your LR5 catalog and converted it to a LR6 compatible catalog that can be used only in LR6. Now you have to return to the original LR5 catalog. Of course, it will not contain any work you did in LR6.