hi every one. am new to mac ox s. have just got a mac book pro. the problem is , i have had it on for two days runing and have just restart it. now the laptop is not even booting up. if i press the ON button to turn it back on, a message will come on the screen that say "you need to restart you computer. hold down to the power button for several seconds or press the restart button". Have done all of the instuction but the same message keep poping up. what can i do? can any body help me with what to do?