I assume youare trying to get miles credited to your BA Exec Club from a CX flight?
So long as you have your BP and ticket stub thingy, you will need to fax them with a form available on the BA website to the partner section. It can take a few months for miles to post. If you have a problem, ask to speak to a supervisor on the phone. If that doesn't work, a letter should.
If you don't the BP and ticket, then you could be in trouble. Theorestically if you can't prove you weren't on the flight, no miles for you evwen though they can check on their own systems.
Good luck.