To provide a general answer for people in the United States, if you signed a document that said your work is a "work for hire", then the client owns the rights. Here is the definition of a "work for hire":
Statutory Definition
Section 101 of the copyright law defines a “work made for hire” as
1 a work prepared by an employee within the scope of his or her employment
or
2 a work specially ordered or commissioned for use as a contribution to a
collective work, as a part of a motion picture or other audiovisual work, as
a translation, as a supplementary work, as a compilation, as an instruc-
tional text, as a test, as answer material for a test, or as an atlas, if the parties
expressly agree in a written instrument signed by them that the work shall
be considered a work made for hire. For the purpose of the foregoing sen-
tence, a “supplementary work” is a work prepared for a publication as a sec-
ondary adjunct to a work by another author for the purpose of introducing,
concluding, illustrating, explaining, revising, commenting upon, or assisting
in the use of the other work, such as forewords, afterwords, pictorial illus-
trations, maps, charts, tables, editorial notes, musical arrangements, answer
material for tests, bibliographies, appendixes, and indexes; and an “instruc-
tional text” is a literary, pictorial, or graphic work prepared for publication
and intended to be used in systematic instructional activities.
www.copyright.gov/circs/circ09.pdf
I am not a lawyer, this is not legal advice.
