如果开机屏幕由黑变成深灰，背部的苹果标志灯亮，但电脑屏幕没有画面，可以试试reset PRAM
Here’s how to reset your PRAM:
1. Shut down your machine. Yes, all the way down, not sleep or logging out.
2. Press the power button and then press command-option-p-r. You have to make sure you get those keys pressed before 3. the gray screen comes up or it won’t work.
4. Hold those keys down until your Mac reboots again and you here the startup chime.
5. Let go of the keys and let your Mac reboot normally.
我也是花128.。。去官方推荐的店检修过mac pro，好在硬件都是好的，但需要重置这个pram （我是电脑小白，也不晓得是啥）。后来习惯了，又自己解决了好几次这个问题。
