Disconnect all accessories.
Hold the Power button for about 20 seconds.
Release it, wait a few seconds, then turn it on again.
Try the Surface UEFI
Shut the Surface down.
Hold Volume Up, then press and release Power.
Keep holding Volume Up until the Surface UEFI screen appears.
If UEFI appears, check whether the internal storage/Windows Boot Manager is detected.
Try Windows Recovery
If it repeatedly boot-loops, allow it to fail several times; Windows may eventually enter Automatic Repair/WinRE.
Choose Troubleshoot → Advanced options → Startup Repair.
If Startup Repair fails, do not immediately choose Reset this PC, especially if important files are stored on it.
If you have a Windows recovery USB, boot from it and use Troubleshoot → Advanced options. We can then check whether the SSD and Windows installation are actually visible before attempting any repair.
If Windows cannot be repaired then use the official Surface Recovery Image for Surface Book 2.