In the United States, Section 106 of the Copyright Act gives the copyright holder a number of exclusive rights, one of which is the right "to reproduce the work in copies or phonorecords." Copyrighted material does not go into the public domain just because the copyright holder has chosen not to publish the material; absent any other exception to the exclusive rights granted by Section 106, the copyright holder still retains the exclusive right to copy the material regardless of whether that material is published or not.
With that said, the fact that a work is out of publication may be relevant to determining whether a particular use of the copyrighted materials falls within the "fair use" exception to the exclusive rights granted in Section 106. There are four factors relevant to determining whether something is fair use:
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
Whether a particular copyrighted work is in print would impact the fourth prong of the test for fair use.