Quote:
I need to boot up from my back up but obviously I can't.
Have you tried starting up while pressing and holding Option? That should give you a choice of startup disks, hopefully including the backup.

Jon

macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365