Duration of Copyrights
Works go into public domain either because: (1) the author failed to satisfy statutory formalities to perfect the copyright; (2) it is a work of the U.S. Government; or (3) the term of copyright for the work has expired.
Item (1) refers to work published prior to March 1, 1989. The copyright notice had to be affixed to the work or it immediately lost protection (the copyright protection for some foreign works has been restored even if they were published without notice before that time) Works published without the copyright notice between 1/1/78 and 3/1/89, the effective date of the Berne Convention Implementation Act, retained copyright only if efforts to correct the accidental omission of notice was made within five years, such as by placing notice on unsold copies. The law has changed so that work published after that time does not need the copyright notice for protection. But it's a good idea to use it, anyway. See my other blogs for additional information.
Item (2) provides that works of government employees, such as maps, charts and surveys, are in the public domain from the date of creation.
Item (3) allows a certain time of copyright protection for the benefit of the creator. The time for that protection has changed several times over the years, so it's difficult to explain when works fall into the public domain. The chart found at http://www.unc.edu/~unclng/public-d.htm is helpful to determine those dates.
Fortunately, copyright protection is easier to obtain and to keep for longer than ever. So do what you can to continue to protect your work. Register your copyrights today.
Take my advice; get professional help.