Bootcamp has created partition for Windows but it doesn't appear under startup disk utility

Hello, this evening I attempted to install windows 10 via bootcamp on my 2019 MacBook Pro 15" following Apple's advice here https://support.apple.com/en-gb/HT201468. I downloaded the windows iso file and the installation seemed to go ok via bootcamp assistant, however when the MacBook restarted (at step 3 in the instructions) it didn't turn back on, and I had to manually restart it via pressing the button. Now I can see that the partition has been created under storage in system preferences, but I can't see it in disk startup nor if I try restarting and holding option. Any help much appreciated.