Press and hold the Power button for 30 seconds until the screen goes completely black. Release it, then press and hold both the Power button and the Volume Up button together for 15 seconds, then release both. Wait 10 seconds, then press the Power button normally. This clears the embedded controller state and is different from a normal restart.
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After two failed boot attempts, the device should automatically enter the Windows Recovery Environment (WinRE). From the Choose an option menu, select Troubleshoot → Advanced options → Startup Repair.
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On another PC, go to Microsoft's Surface Recovery Image download page and download the recovery image specifically for the Surface Pro 8. Flash it to a USB drive (32GB+, FAT32).
Boot your Surface using Volume Down + Power, then choose Troubleshoot → System Restore or Startup Repair to try fixing it without wiping your file