Last night I tried to wake my Macbook, as usual, from sleep by typing in may password. The computer stayed on the lock screen. Eventually I decided to reboot, but then the progress bar at startup would reach about 1/3 and just hang. I tried the following (in this order): * PRAM reset. Did not help * Reinstalled Yosemite - it took forever (almost 90 min) and still would not start (1/3 progress bar) * Eventually tried a restore from Time Machine. That ran overnight but seems to have fixed the problem. Does anybody know: * What might have caused the initial crash? * Why a re-install would not work? * Another (quicker) way that I might have tried to get my system back?