Boot Manager not Working?

So I've been really enjoying my Steam Machine so far, steam games run great, download speeds are insane, overall an amazing purchase. If you are unsure about getting one, just get one asap.

But for reasons I cannot go too much into details here, I need to access the Boot Manager.

I've tried to tap escape as the machine boots up, I've also tried holding down the power button until the light turns white, then pressed it again when it turned either purple or yellow. The result is the same, my monitor detects an hdmi output, but the screen remains completely empty / black as the LED strip breathes slowly in blue indicating that the SM is booting up.

I am very confused, I haven't installed anything on the computer other than five games, discord, and a web browser, so surely there must be some kind of setting or system thing that isn't working correctly.

Has anyone else experienced this? I know people have managed to install other OS and some even dual boot despite the lack of official support for it, so surely I must be missing something. Please help if you know anything!