There are several ways of booting a Windows system into Safe Mode. But if you're no longer able to boot into the system to turn on Safe Mode, and the system doesn't automatically boot into safe mode, you're usually only left with a single option: using a Windows Installer USB or DVD/ISO to enable Safe Mode.

To accomodate this, you'll need a bootable USB-key, DVD or ISO image (if you're running a VM).

1. Start up the system, booting from the bootable media. Because you already have Windows installed, you will need to hit Enter in time to boot from the bootable media.
2. When Windows Setup has started and the button "Install now" appears, click on the Repair your computer link at the bottom left corner of the window.
3. Click Advanced options → Command Prompt, which will open a command prompt.
4. In the command prompt, type in the following commands to enable the boot menu, and optionally set its boot timeout to 15 seconds (the default is 30 seconds).

   bcdedit /set {bootmgr} displaybootmenu yes
   bcdedit /set {bootmgr} timeout 15

5. Reboot the server. You will now be greeted with a boot menu, usually with a single entry and with a countdown timer of 15 seconds. Hit F8 on your keyboard to go into the advanced boot menu options, then choose Safe mode to boot into Safe Mode.

Note that if you wish to undo the changes and hide the boot menu, you can run the following command in an elevated command prompt (Powershell does not work here because of the curly brackets):

bcdedit /set {bootmgr} displaybootmenu no