Q596: How can I trace a missing relative with whom I have lost contact?
There are several organisations that will be able to help you trace missing relatives. The main one is the Salvation Army who have contacts in over 90 countries.
If the separation is due to some form of conflict (political, natural disaster or armed) then the Red Cross may be able to help.
If it is due to children being abducted by an estranged parent, Reunite will be the appropriate organisation.
Perhaps 192.com may help if you have enough details about the person.
See links in related information for more details.