how to find someone missing

[Taz6]

Im trying to find a member of my family which we lost contact with, about 12 years ago.

[Whoever]

Wite to their last known address with a request on the envelope for the occupant to forward to them if their new address is known. Put your home address on the back of the envelope.
Check if they have listed themselves on Friends Reunited.
Try putting their name into Google and see if that brings up any result.
If you knew where they worked, you could try sending a request to their HR Department to forward to their home address.