First, press and hold the power button for about 15-20 seconds. Keep holding it until the screen goes completely dark. Once it's off, just wait around 30 seconds, then press the power button once and let it start up like normal.
If that doesn't work and it's still acting weird, here's the backup plan:
Turn it off completely, then hold down the volume up button. While you're still holding volume up, press the power button. When you see the logo pop up, let go of the power button but keep holding that volume up button. Don't let go until you see the recovery or UEFI screen show up.
Once you're in the recovery screen:
Look for "Troubleshoot" and select that. Then go to "Advanced options" and click on "Startup Repair." It'll ask you to pick your Windows account, so choose yours. If it prompts you for that BitLocker key again, just enter it one more time.
This should fix a bunch of common issues like corrupted boot files, Windows updates that went sideways, or when your Surface's firmware gets stuck.