Aetna just denied in-network 3x for PPND-717 – We asked you or your provider for more details, but we didn’t receive them.

I was just denied by Aetna for IN-NETWORK coverage for over $2,000 worth of medical bills because of the following code:
717: We asked you or your provider for more details, but we didn't receive them. We need the information before we can fully review the claim. [PPND – 717]

What does this mean? What am I supposed to do about this?