That means, are there any such conlangs that exist today but were once declared as dead or not in use by any community?
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.