Upvote:2
You should be allowed to board, as NICOP is an official identification document, a visa, for Pakistani citizens living and/or working abroad. A test search through Timatic, the database used by airlines to check passenger document requirements, indicated that boarding would be permitted if a passenger holds a certificate of citizenship for Pakistan.
That said, it would be wise to carry with you a printed copy of the FIA Ministry of the Interior brochure which states that you would be allowed to enter even if the NICOP has expired.
Should you have sufficient time before departure, you may want to contact the nearest Pakistan Mission, or begin the renewal process online.