Mon Colle Knights (Possibly lost anime English dub master tapes; 2000)
Mon Colle Knights started out as a Japanese manga series written by Satoru Akahori and Katsumi Hasegawa, and illustrated by Hideaki Nishikawa. An anime series was produced by Studio Deen, airing from January 2000 to December 2000, consisting of 51 episodes and one movie. An English dub produced by Saban Entertainment aired on Fox Kids in North America from July 2001 to September 2002, which consisted of 45 episodes. In 2006, it aired on reruns on Toon Disney's Jetix block, but after it got cancelled, Mon Colle Knights has not gotten a broadcast or home video release.
While the English dub does exist, as videos of all the episodes are available on YouTube, the English dub master tapes are supposedly lost. An anime licensing company, Discotek Media, acquired the license to the anime, but at this point in time, only have access to the original Japanese version, which is available for streaming on Crunchyroll as of June 8th, 2020.
According to some (now deleted) Twitter posts by Anime News Network founder and Discotek employee Justin Sevakis, https://www.animenewsnetwork.com/interest/2020-06-11/discotek-turns-to-community-for-help-locating-mon-kolle-knights-medabots-english-dub-masters/.160491, the Fox Kids dub masters cannot be found anywhere. There is speculation that Disney has the master tapes, but so far, nothing is concrete, and Disney is notorious for hoarding various shows even if they don't air them anymore. As of September 15th, 2020, Discotek is still searching for the master tapes, as editing the entire dub from scratch to match the high quality footage is financially impossible for them.