Q: computer won't start up after mavericks update

Startup your Mac while holding down the Option key.

That should launch the Startup Manager window where you can select the startup disk then click Restart.

If that doesn't work, try a Safe Mode boot.

Startup your Mac in Safe Mode

A Safe Mode boot takes much longer than a normal boot so be patient.

Once you are in Safe Mode, click Restart from the Apple () menu.

See if your Mac boots normally.

Same thing happened to me.

Boot up holding (Command S)

< Screen will be black, when you see the lines of code end, you can start typing >

*After every line hit return*

mount -uw /

cd /Library/Preferences

mv -r com.apple.loginwindow.plist oldlogin

reboot

My appologies, I made a typo in my previous post.

After the lines of code end type the commands below and hit return after each line.

___________________________________________________

mount -uw /

cd /Library/Preferences

mv com.apple.loginwindow.plist oldlogin

reboot

___________________________________________________

After the computer reboots, wait a few moments and click around on the mouse.

Hope this helps.

You sir, rock. Thank you for saving my mother's and, by extension, my bacon.

http://support.apple.com/kb/HT1492

Startup Issues - Resolve