After updating to os x yosemite, the OS refuses to start. What are the solutions?

Question

macOS

It just displays a crossed out circle on a black background.
Updated via appstore, no OS other than os x is installed.
Has anyone experienced this? How can I restore the OS without losing data?

Answer 1

I solved the problem:
- cmd + r at startup
- reinstall os x
After that, the installer still swears, but now, if you turn off the device and turn it on again, the OS will start.
We refuse to configure, just turn off everything on the screen, save all the important data to a USB flash drive and do a clean installation .....
Of course, I did not expect such a setup from apple ....