Contact the department for work and pensions.
i'm sorry but this made me laugh cos i was in the same situation as you a while back so i know how you feel. Basically my job centre told me that if u have it written down somewhere than its not a problem but if you dont then you do need to contact your local jobcentre to get it replaced.
I lost mine recently.
If you've had a National Insurance number but you've forgotten or lost it, you can obtain a replacement number card by contacting your local Jobcentre Plus, Social Security Office or Jobcentre. You can find the telephone number in your local telephone directory.
For more information you can phone the National Insurance registration Helpline on 0845 915 7006.
I phoned up the national insurance contributions office
they sent me a note of my number (as I hadn't recorded it anywhere)
they also sent me a form to fill in for another card -
"for secrity reason, the issue of replacement cards is now resticted. you are only entitled to receive one original and one replacement. When we issue the replacement care you will not be ble to receive any more, even if your circumstances change."