From the BBC:
On 19 December, 1995, Diane Lilburn walked out of her home in Lincolnshire. She left no note for her mother and stepfather, she simply disappeared.
Her parents spent the next six years trying to find her, but with no sightings or leads their search was ultimately fruitless. Her mother Peggy had to try to come to terms with the fact her daughter might never come back.
In a last-ditch attempt Peggy contacted the National Missing Persons Helpline (NMPH), after learning about the charity from a television programme. It found her daughter within a month, tracking her down to Brighton.
Diane was reunited with her family and told them she had run away from home because she was gay.
The NMPH does an amazing job. The BBC article states how, unlike police, NMPH has no access to things like mobile phone records, bank records, etc. But they generally find 70% of missing people.
Information on missing persons organisations can be found here.