Several possibilities (I assume other D notes on the instrument are OK, just this one position is dead); the first thing I would look at is the 7th fret. If it's loose in the middle that could be the culprit. Tap on it with the end of a 6" metal ruler- if you get a high clear 'tink' at the ends but a lower 'tunk' in the middle it's loose and absorbing energy. Wick a little thin cyanoacrylate along it while holding the fret down tight.