Hi Matt,
Thank you for posting your problem we are here to help you. Possible reason why your Surface Pro 9 suddenly shutdown is Firmware or Software glitch, corrupted updates or battery issue.
For now please try this steps check if this will help to fix the problem.
Try to connect the Surface Pro 9 charger to different power outlet then test if will work.
Disconnect all cables connected to your Surface 9 Pro including the charger then turn on your Surface Pro 9 to test.
For more details check this article from Microsoft: Surface won't turn on or start
To raise a request for repair service try this link.
Open Web browser
copy and paste this Microsoft Account | Device link:
https://account.microsoft.com/devices/select-device-for-repair
Log in using your Microsoft account and Select the device that needs service and then select Next.
If it isn't registered, select Register device. Next Follow the instructions to complete your service order.
You can check this article from Microsoft for further details: How to get service repair.
looking forward to helping you out. If you need more help please feel free to let me know.
Best regards,
Jansher