title_body: Is there an existing constructed language that once was dead? That means, are there any such conlangs that exist today but were once declared as dead or not in use by any community?
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.