(HOW TO FIX PC ERROR)
Click start, type into the search bar %appdata% click Roaming, now click .minecraft and copy it to your desktop (this is backup). You can also do this for your saves. Once you've done that go back to .minecraft (In roaming) and delete it. Now open minecraft click options and select FORCE UPDATE and login. It will redownload everything. To get your old saves back Copy and paste SAVES into %appdata>roaming>.minecraft and delete the NEW save folder. Hope this helped, sorta.
