My wireless keyboard is flashing green but won't show up on my mac.

I switched off my computer last night but this morning can't log in.

My Bluetooth's keyboard is flashing green, which means it's discoverable.

I did find a question here on the forum on the same topic but in that it says go to System Preferences and so on, which obviously I can't do because I need to type in my password to log in but the keyboard isn't working.

any tips?