As with all updates Apple recommends making a backup of your drive just in case something like this goes wrong. Hundreds of thousands of computers updating will have an error or few along the way.
If nothing else, try booting up to your system disk and enter Disk Utility, select your drive and attempt the repairs offered. If it does not work, then you can always do an Archive and Install.
While booted to a system disk click continue until it asks you to select a destination for installation, click the options button and select ARCHIVE AND INSTALL and PRESERVE USER DATA AND SETTINGS. This will reinstall the base OS from scratch, setting the old system aside as a Previous System (for your later review and deletion) and preserve your user settings and home folder. Then, you'll have to update the system back up through the Software Updates.