Middle English

  Old English did not disappear overnight at the Norman Conquest but in the years following the Conquest, changes which had already begun to show themselves in pre-Conquest Old English continued. Norman Conquest of Britain by William, the Duke of Normandy occurred in 1066.  The conquest was followed subsequently by the introduction of new spelling conventions. Norman scribes disregarded the traditional English spelling and simply spelt the language according to what they heard it, using many conventions of Norman French. Consequently, many changes which had not been reflected in OE spelling, or which had appeared only in occasional spellings, now emerged clearly.  

  A number of new consonant symbols were introduced. A new symbol g was introduced for the stops represented by OE ʒ, and the OE symbol only retained for the fricatives. OE had used f to represent both [f] and [v] whereas ME scribes used u or v the voiced sound. Similarly, z was introduced by s , though not consistently. The digraph th gradually replaced þ and ð, but ð was found up to 1300 and þ remained common until about 1400. It is to be noted that in Middle English there were separate phonemes /f/ and /v/, /s/ and /z/, and /θ/ and /ð/, where in OE there were pairs of allophones. 
  
  The letter y was no longer used as to represent a front rounded vowel, but was simply used as an alternative to i, so that ME king and kyng represent exactly the same pronunciation as do fir and fyr 'fire'. OE [dʒ] never occurred in word-initial position, only medially and finally. However, ME loanwords from French, like judge have [dʒ] in initial position.

  One oddity of ME spelling was the result of change of script. Norman scribes used a continental style of handwriting to replace the insular script of OE. Continental style made it difficult to tell how many strokes had been made when letters like m, n and u occurred together and hard to distinguish  between groups like un, uu and um. In such case, scribes often wrote o instead of u. Thus, we often find ME sone, comen and loue(love) for OE sunu,
 cuman and lufu.

 Source: Charles Barber, Joan C. Beal, Philip A. Shaw The English Language A Historical Introduction Cambridge Approaches to Linguistics  2009