Is milk sour or did it spoil? Its a big difference! While soured milk is perfectly edible, spoiled milk can be toxic!

If it doesn't have foul (not rotten) odor, yellowish and slimy appearance (trust your nose and your eyes!), it is soured milk and you can make cheese with it. Harzer for example, is cheese made from sour milk. 

Or you can make cottage cheese if you prefer. Check this [link.][1]


  [1]: http://www.theselfsufficienthomeacre.com/2013/02/cottage-cheese-made-from-sour-milk.html