That means, are there any such conlangs that exist today but were once declared as dead or not in use by any community?
2 Answers
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.
My answer is a little biased, but I can speak for the history of Solresol. Solresol was invented in the early 1800s, grew in popularity over the next ~70 years, even after the death of its creator, and then (apparently) abruptly died out in the early 1900s, despite supposedly being at the peak of its popularity. There were only a few brief mentions of Solresol throughout the 1900s in books about universal language attempts. Then, in the 1990s, someone started a mailing list discussion about Solresol through reading one of those books, and the community has been slowly growing ever since, with a desire to further develop and promote the language.
Gufferdk makes a good point that it's difficult to prove the "death" of a conlang. There is no known documentation of Solresol being in use through most of the 1900s, but I cannot prove that more private communication did not exist.