Enter Windows 8 Safe Mode when you can't boot Windows 8
If Windows 8 won't boot normally, it probably won't boot into Safe Mode, either. You can try the trick of repeatedly pressing and releasing F8 (described in my previous article), but it seldom works with Windows 8.
So what can you do You have to fix the problem that's keeping your PC from booting at all. Then, once Windows 8 can boot, you can reboot it into Safe Mode (if you still need to).
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The trick is to create a recovery drive, and boot from that. It's best to do this on your PC before you have a problem. A recovery drive created on another Windows 8 PC might work, but it might not.
Follow these instructions. You'll need an empty flash drive, or at least one that contains nothing that you want to keep.
Try booting from the flash drive. You may need to go into your BIOS setup and tell it to try booting from the flash drive before it tries booting from the internal drive. Your computer almost certainly uses UEFI, which in most cases isn't a problem because it probably supports BIOS boots as well.
Once successfully booted, the recovery drive will ask you to select a keyboard layout. On the next page, titled "Choose an option," select Troubleshoot.
On the Troubleshoot page, select Advanced options.
Once there, try Startup Repair. If that doesn't work, try System Restore. Chances are that one of these will work, and when it does, you will probably not need to enter Safe Mode.
In the worst-case scenario, you'd have to use System Image Recovery or reinstall Windows. If you have to do one of these, Safe Mode wouldn't have helped you anyway.