On certain devices it can appear to be hard to get into Safe Mode in Windows 10, e.g. Microsoft Surface Pro 4. If your device has failed to the point of not booting Windows and the F8 key is not working then if you can get a Windows 10 recovery drive set-up you can boot your device, once you have selected Language and Keyboard options select Troubleshoot > Advanced options > Command Prompt. At this stage if you have a bitlocked hard drive then you will be required to enter you key.
Once done you can gain access to the C: drive, then enter the following in the command window
Then select Turn off your PC and the boot your device and repeatedly press F8, this should hopefully get you into a similar set of screens to the recovery drive but this time there will be an option to access boot options which include Safe Mode.