In the field of linguistics, a living language is one that is used by a community of speakers who learned it as their first language. For example, these are all living languages: English, Modern Greek, Mandarin Chinese, Navajo, Tibetan, Swahili. As a counter-example, bureaucrats at the Vatican speak Latin, but it is not the first language for any of them. For this reason, Latin is not considered a living language in spite of its still being spoken by a small community.
Whether or not a language exists in written form is irrelevant to its status as a living language. In fact, a majority of the world's 6,000 living languages are not written by their native speakers (altuough some outsider, for instance an anthropologist or linguist, may have devised a writing system.) Writing is always an imperfect representation of speech (because not all linguistically relevant details, such as the intonation of sentences or the accent of the speaker, can be written.) Hence, linguists consider spoken language primary and writing secondary. But the only way to study languages of the past (like Sumerian), or past stages of existing languages (like English), is through the writing that has come down to us.
Before the invention of printing (ca. 1450 in Europe; printing was already in use in China) texts had to be copied out by hand. We almost never have the manuscript that was written by the author himself (or herself), because the odds are against such a perishable object existing for such a long time.
For instance, there are no extant manuscripts of The Morte d'Arthur written by the Middle English alphabet
The Middle English alphabet is like the Modern English alphabet, with some exceptions. For the purposes of the corpus, every letter of the Middle English alphabet was assigned some equivalent in the ASCII code, as described below.
æ in Early Middle English appears in place of several vowels that would later be written "a" or "e"; it is represented in the corpus by +a.
‰ usually reflects the sound later written "gh"; the sound has disappeared from Modern English but the letters remain. For instance, "right" in Modern English, in the Middle English period was pronounced "richt" and written "ri\137t". \137 is represented in the corpus by +g.
ð and þ are both used to represent the sounds later written "th"; they are represented in the corpus by +d and +t respectively.