I've heard examples of people speaking in Old & Middle English, they sound so far removed from each other, and from modern English. Did the changes happen gradually over centuries, or were their sudden shifts in the way the language was spoken & written? What factors led to the changes? Were changes localized or wide spread?
Two caveats, dialects and detail. Firstly, a lot of things happened to change the language along. I can only mention some here. Secondly, there were a large amount of dialects in Old English, although the West Saxon dialect is most heavily represented in the source material. Most poetry and prose is in West Saxon, although examples certainly exist for Kentish, Northumbrian and Mercian. It is likely that in literary expression, as for today's RP, West Saxon was the appropriate form of Old English to use. Dialect division along these lines was prominent right through the Old, Middle and Modern Periods. When I talk about language change below, I'm talking about culturally dominant dialects.
Middle to Modern English was a question of some phonological change (note especially the Great Vowel Shift), regularisation of orthography (spelling - this was awful before the Early Modern period), a more modern process of word-formation and a further simplification of grammar (like the loss of thee, thou, etc.) One of the most important changes, however, especially for a Middle English person listening to Modern English, was the huge amount of lexical borrowings. One of the defining characteristics of modern English is both the large amount of loanwords taken from former provinces of the British Empire (khaki, pajamas, hooligan, etc.) and those Latin and Greek words invented for the purpose of science and other endeavors of the Enlightenment period. Some of the latter came in during Middle English (client, conviction, dissolve, recipe, conflict, imaginary) and others in Early Modern English (genius, species, radius, apparatus, focus, tedium, lens, area, alias, offensive, circus, album). We might also note the influence of early writers, Shakespeare as the most famous inventor of words in the modern age.
Old English to Middle English is a process that has been described before in this forum. Suffice to say, huge grammatical simplification (basically the destruction of the whole case system) and a massive borrowing from Norman French. 1066 and all that.
For further information, I'd recommend getting Barber, Beal and Shaw's The English Language: A Historical Introduction, 2nd Edition (2009). It's very good.