Force it fully off: Hold the power button for 20-30 seconds (even if the screen already looks black) until you're sure it's completely off, not just sleeping or hung.
Wait 10 seconds, then do the UEFI key combo from a cold start:
Press and hold Volume Up
While holding it, press and release Power
Keep holding Volume Up for 10-15 seconds until the Surface logo appears, then let go.
If the above suggestions fail, then download: Surface Recovery Image Download and Reset or Recover.