title_body: Is there an existing constructed language that once was dead?
upvoted_answer: Since conlangs by their very nature don't start out with any speakers, and in most cases the author is not immediately a skilled user of the language, conlangs tend start out dead, and so under this definition almost any conlang with a speaker community would technically once have been dead.
  
  However, going by "dying" as having a community, then losing it, reliable data is somewhat hard to find, as a small remainder of a community (which could be as little as two individuals' private postal correspondence) can be very hard to locate especially in pre-internet days, though it seems that Idiom Neutral, published in 1902, once had largely fallen out of use, but has recieved a recent revival by enthusiasts in a couple of internet usergroups and blogs.