From the Wikipedia page  which has a list of writing systems and links to the specific articles.
Many undeciphered writing systems date from several thousand years BC, though some more modern examples do exist. The term "writing systems" is used here loosely to refer to groups of glyphs which appear to have representational symbolic meaning, but which may include "systems" that are largely artistic in nature and are thus not examples of actual writing.
The difficulty in deciphering these systems can arise from a lack of known language descendants or from the languages being entirely isolated, from insufficient examples of text having been found and even (such as in the case of Vinča) from the question of whether the symbols actually constitute a writing system at all. Some researchers have made claims of being able to decipher certain writing systems, such as those of Epi-Olmec, Phaistos and Indus texts; but to date, these claims have not been widely accepted within the scientific community, or confirmed by independent researchers.
The Voynich Manuscript appears to be an undeciphered writing system, but may be an uncracked code or a hoax document. As it is a unique example of the usage, without an indication of the context in which it was created, it is impossible to decide. However it does resemble medieval writing in general of the time.