Hello Robert Butterton,
Thanks for reaching out to Microsoft Community. I'm sorry to hear about this issue on your device. I understand how frustrating it is to have such problem. Definitely, not the experience we want you to have.
If I may ask, have you set up BitLocker prior this issue happened?
When was the last time you updated the device? Does this issue happen after an update perhaps?
When the issue happened, were there any peripherals connected to your device?
In the meantime, try Finding your BitLocker recovery key in Windows - Microsoft Support and enter it. Otherwise, try forcing your device to shut down and restart. Press and hold down the power button until your Surface shuts down and restarts and you see the Windows logo screen (about 20 seconds), then release the power button.
Let me know how it goes. I'll wait for your response.
Regards,
StevenK