India Post — Registered letter from usa got lost in delhi and did not reach kolkata address

Address:

New Delhi, Delhi

We sent a registered mail from charlotte, north carolina, usa on 22nd feb, 2016. It was delivered to indian post office at indira gandhi intl, delhi on 28th feb, 2016 @ 22:32 pm. The registration no. Is re[protected]us.
We went to kolkata gpo to get the status of this mail and came to know that whole package did not come to kolkata. At this point we have no clue what to do and it is carrying very important document for us, losing it would cost lost of money and work progress with indian income tax department.

Appreciate your immediate attention on this issue.