|(Assuming desktop) If you are using a wireless keyboard, try a USB wired one. Wireless keyboards are not active until Widows loads.|
If using a USB keyboard but it is not connected directly to a rear USB port on the motherboard directly try it. USB ports that are connected through a hub, even an internal hub which includes many front USB ports are not active until after Windows loads.
If you have a machine that uses USB2 and USB3 ports, try the USB2 port. Many early USB3 ports were run through third party chipsets that are not recognized until Windows drivers load.
(IF Laptop): Try a USB external keyboard.
Otherwise you are probably going to have to reset CMOS through jumper to get in to reset BIOS.
You have to be a little bit crazy to keep you from going insane.