Hello Aoife Moroney,
Welcome to Microsoft Community.
From your description of the problem, I notice that you should have already performed the steps to force a device reboot to shut down:
Force a shutdown and restart your Surface - Microsoft Support
But let me kindly remind you that this operation only requires you to hold down the power button for about 20 seconds, and does not require you to hold down the volume up button while you press the power, because the double-key startup step does not work with the Surface Pro 8.
If the problem persists, I may need to recommend that you reset your Surface using a Surface-specific USB recovery drive, as shown in Solution 3:
Surface turns on and shows "Couldn't find a bootable operating system" - Microsoft Support
You will need to create a Surface-specific recovery drive in another device that is available.
If you're still out of luck, you'll need to follow the following guidelines to send your Surface device to the Surface Support team for repair:
How to get service or repair for Surface - Microsoft Support
I wish you all the best.
Best Regards,
Mitchell | Microsoft Community Support Specialist