The answer is that the Book of Enoch, also known as 1st Enoch, certainly has not been lost, and neither has it been forgotten. It was not taken out of the Old Testament because it was never part of the Old Testament. Anyone who likes can read the translated text any time they like. Here is a link to the text of the Book of Enoch: http://scriptural-truth.com/images/BookOfEnoch.pdf
In fact, the Ethiopian Coptic Church still has the Book of Enoch in its Old Testament canon. This is the only significant Christian group which includes Enoch. It has been in their canon from ancient times, so the book has never been “lost.” It was “lost” to Europe from the early Medieval period until the late 1700’s when a Scottish explorer James Bruce brought three manuscripts to the UK, from which it was translated into English in the nineteenth century.
So, what is the Book of Enoch, when was it written, by whom, and why is it not in the Christian Bible (except for the Ethiopic church, in which case, the question might be why is it included in theirs)? The Book of Enoch is a pseudepigraphal Jewish text of the second or third century BC. Whether the original language was Hebrew or Aramaic is not clear, as the oldest text of the Book of Enoch is actually in the language Ge’ez, which is the ancient Ethiopic language. The book is said to be pseudepigraphal because the book is ascribed to the same Enoch who was seventh from Adam. Obviously, that Enoch did not write this book, so it is falsely attributed to him, which is the meaning of pseudepigraphal (falsely signed). To modern minds it is deceptive and unethical to falsely attribute something we would write to a more famous person, but in the ancient Near East this was considered an acceptable practice. 1st Enoch is written in apocalyptic style, with visions and symbols, as, for example, Revelation or Zechariah. The book was known to the Jews in the first century BC, as is proved by their quotes from Enoch, by its having been found among the Dead Sea Scrolls, as well as from the fact that the New Testament book of Jude (Jude 1:14-15) mentions the story in the Book of Enoch. Whether Jude is simply using a common story to make an illustration or he is attributing inspiration to the book is not clear from how it is used in Jude. Early church fathers also mention Enoch and occasionally quote from it in Greek translation. Origen, Tertullian and other early church fathers mentioned and used the Book of Enoch, and there is good evidence that it influenced their thinking about the Old Testament. For obscure reasons, the church in Ethiopia was particularly fond of the Book of Enoch; so much so that they included it as part of what we know of as the Old Testament Apocrypha. Some in the Ethiopian church go so far as to claim that it was originally written in Ge’ez, which is almost certainly not true.
As for the content of the Book of Enoch, it includes Jewish stories which provide added details to Old Testament texts. For example, in 1 Enoch we learn that demons are fallen angels and that such fallen angelic beings had sexual relationships with human women, giving birth to the nephilim. It contains descriptions of Sheol and the “underworld,” it names the seven archangels (only Michael and Gabriel are named in the Bible). Enoch also contains descriptions of the heavenly objects and their paths. It uses a solar rather than a lunar year, which is an interesting curiosity. Enoch is divided into five parts, including the Book of Watchers, the Book of Parables, the Astronomical Book, the Epistle of Enoch and the Names of the Fallen Angels. The text is rather long. You can see in the link above that the text in that format is nearly 150 pages long!
Although 1 Enoch was popular in the early phases of the Essenes at Qumran, and although some Jews clearly read and valued the book, it was never received into the Jewish canon. The situation is similar in Christianity. It is clear that the early church highly valued the Book of Enoch, enough to quote from it and to have their understanding of the Old Testament be influenced by it. However, there is no evidence that it was seriously considered to be included in the Old Testament canon by either Western or Eastern Christianity.
Is there any chance that some of the knowledge found in 1 Enoch, particularly about the archangels and the source of the nephilim is in fact true? It is hard to judge, as the inspiration of this collection of writings is very dubious. I would say that it is best to read the Book of Enoch, understand its role in forming Jewish and even Christian thought, and to leave it at that. We should not do what the early church did, which is to allow it to influence our view of God.