Yeah, that startup repair is a good first step. If it doesn't work, try going into the Troubleshoot menu and using the "Go back to the previous version" option to revert to Windows 10 for now.
Yeah, Agreed, that revert option is your safest bet. Also, if you can get into the BIOS, double-check that your drive is still set as the first boot device. Hope that helps.
Can confirm. You could also try booting from your Windows 10 install media and running the startup repair tool. Another option is to check if your storage drivers need an update, especially if you're using a newer NVMe drive. Let me know if that works.