Currently having probelms with my 15.4" MacBook Pro MD103B/A currently running Yosemite OSX Beta. When powering on the Macbook the start up sound plays, and the screen is extremely dim, I can just make out the loading bar and then my user profile picture and password box. Entering the password and then pressing enter doesn't work. The keyboard does not function as I have tried increasing the back light strength using F2. Any suggestions on what I can do?