The National Missing and Unidentified Persons System (NamUs) maintains an active database with information about missing persons, which you can access and search here.
The information on the NAMUS site is updated daily. Law enforcement agencies and medical examiner's offices from across the country share this information with the hopes a missing person can be reunited with his/her family or unidentified remains can be sent home and given a proper burial.
Anyone with information helpful in locating a missing person or identifying an individual should contact the law enforcement agency listed for that specific individual.
If you do not see your missing person on the national database, you may register on the website as a public user and fill out a case to submit to the database. This information will be checked with law enforcement and then published on the website.