If you have an old boarding pass it might have a OnePass number on it (if you have one). That's a 8 character string with 2 letters followed by 6 numbers.

If you can't find and old boarding pass call Continental and ask them if they can try and find you in the system by name/address.

Then go to continental.com and get a log on for your existing OnePass account.

If they tell you that you don't have a OnePass account then you are probably stuck - old trips can only be credited to an account for (6?) months after the flight.