Medieval Latin is somewhat different from Classical Latin because it had ceased to be a living language. If someone who never learnt Classical Latin wants to learn Medieval Latin on their own, which resources can or should they use? Are there any resources for studying Medieval Latin from scratch (i.e. without previous knowledge of any version of Latin)? If yes, which resources are available? If no, would it be feasible to first learn Classical Latin and then somehow move on to Medieval Latin?
Medieval Latin is not a very well defined language. It is basically somewhat successful attempt at keeping Latin alive for official and liturgical purposes in the Catholic Church. Since most of their source documents were already written in Latin or translated into Latin during Roman Empire era, they decided to keep it even when the country got conquered by Germanic warlords. The language was slowly evolving under the influence of barbarian languages so the church documents from different eras are written in different versions of "Medieval Latin".
In either case, if you live in a Catholic country try your local seminary. They still teach Latin to priests (although most of them probably sleep through it), both the Classical and the Late versions. Church documents are also good source of reading material for practice once you get the basics.
Learn medieval Latin is an online resource created by The National Archives in the UK. The description on the homepage says,
This step-by-step beginners’ guide to medieval Latin, created by our experts, will help you gain the necessary skills to read documents from this period.
Mittellateinisches Glossar by Edwin Habel and Friedrich Gröbel (Schöningh, 1989, 431 pages) is a Latin-German pocket dictionary with 10.000 entries.
The book Einleitung in die Lateinische Philologie des Mittelalters by Walter Berschin (Mattes Verlag, 2012) is the published version of a lecture series about Latin philology and may serve as a complement to the site "Learn medieval Latin", rather than as a language learning resource in its own right.
Apprendre le latin médiéval. Manuel pour grands commençant by Monique Goullet and Michel Parisse (Picard; first edition, 1996, second edition, 1999; 215 pages) is a French textbook.
(Some of the above resources are in German; universities in Germany that teach Medieval Latin include the Ruprecht-Karls-Universität Heidelberg and the Friedrich-Alexander-Universität Erlangen-Nürnberg.)