People go missing in Thailand on a regular basis. More often than not, it seems they have simply neglected to stay in touch and are unaware that people back home are worried about them and usually turn up unharmed. If your loved one seems to have vanished into thin air and all attempts to contact or trace them have failed, you can post a missing person announcement for free on this site
You should provide as much pertinent information as possible, including their last known location, who they were with, their destination and their itinerary. A photograph of the missing person would also be a good idea.
Note of caution
Apparently some people who have listed family members as missing have been contacted by an individual who claims to know where their loved ones are and asks for money for the information. If that happens, you can go to this page
and the site organizers will deal with it.