I already downloaded and intall Yosemite OS last year 2014 when it was available and it was good, but to my surprise today April 2015 when i saw an update available again for Yosemite OS after it was installed the update, there was an error and i cant access my macbook anymore every time i log in my password on my account it will restart all over again and again, i dont know what happen, for now i was able to send this concern using guest account on my macbook and its only limited application to view only safari browser available for guest account, pls help me to solved these problem of mine.