Details, please. What are you trying to do to "access" these photos? Please use Lightroom words to describe the steps you are following, as "access" is not a word used by Lightroom.
If you can only see one of them, which one can you see?
Was the DNG and the TIF imported at the same time?
How are you trying to "access" the files?
The more details you can provide, the more likely it is that someone will be able to help you.