Mac Book Air Mid 2013: Mavericks: after some upgrade (was done automatically) reboot did not work !!! found solution for me

• using recovery and repair disk and repair permissions did not help.
• selecting boot device did not help.
• booting into safe mode (shift) did not work.
• booting into single user mode (CMD + S) worked.
• booting with verbose mode (CMD + V) stopped loading at:

var/db/BootCaches.playlist 2:no such file or ...

searching with google I found an entry which had the perfect solution for me.

Boot single user mode. CMD+S.
Follow instructions to mount disk in write mode.
Delete etc/launchd.conf
and reboot.

That worked like a charm.

I almost wanted to reinstall from Time Machine Backup. Many thanks to cgomezsantos!!!!!!

https://discussions.apple.com/thread/5737729