Why did my Mac reboot into the "Set up your new Mac" screen after a power failure? I accidentally turned off the power strip that my iMac was plugged into while it was still running. When I turned it back on again, it put me at the "Set up your new Mac" screen, asking me to accept the Terms and Conditions and create a new user account.

I held off on doing so for a while because I was afraid that setting up a new user would erase my home folder and thus all my data. After trying several times to boot normally and failing, I eventually gave in and created a new account under a fake name.

Once I was in, I was able to select and log in to my original account from the drop-down menu on the top right of the taskbar. Thankfully, my original user account and all my data was still there.

What happened? Why wasn't I able to just boot normally after the power failure?
At boot, it was having a problem in finding any users accounts, so it offered to create a new one.

In process of doing so, it repaired it self when it found out there are already users.