Why the heck did they remove the F8 solution at first startup?????
I don't know, but you and I and a whole bunch of other people wish they hadn't.
I think it's because some motherboard manufacturers or system manufacturers have usurped the F8 key for their own usage.
IF you have the installation media you can try it from there
Boot into the Windows 10 Recovery Environment either by using a Recovery Disk that you've made, or the original Installation Media.
Choose Repair, Advanced Troubleshooting, Command Prompt and type
bcdedit /set {default} bootmenupolicy legacy
reboot your PC and tap the F8 key (no shift key required) and it should work for you as it has in the past.
Now to return your boot menu to normal
bcdedit /set {default} bootmenupolicy standard
You might also try the "Startup Settings" and or a "System Restore" from the available Advanced Troubleshooting options