I too need an urgent fix for this. I have made a bootable installer but start up disk refuses to see it so i cant even do a clean install. There has to be a service or something it cant get past. BTW MacBook Air retina. Are we all on the same machine?

Same here. I can however boot into safe mode and it shows I am on BIG sur. I assume its down to an extension or plug in but I have been through all of them and no joy. In normal mode I get to 5% approx loading then nothing need a fix urgently