Did you back your system up using "Time Machine"? If so, restart and press the "Alt/Option" and keep it held. This should show the volumes you may boot into. Now, it's been a while since I've had to do this, but I believe with 10.7 Lion things changed. I believe you need to boot into the Recovery partition which will display the backup points and the OS with each backup. Choose a backup with 10.8.x, select the drive, you may be warned that the drive will be erased and restored with the chosen backup. Let it run, reboot, and done.
If you do not have a backup, you may be able to reinstall 10.8 over 10.9. I'm not certain. You may have to get the InstallESD.dmg from the contents in the 10.8 installation file. If you right click on the 10.8 installation file, open "Contents", then "Resources", then "Shared Resources" you should see a file named "InstallESD.dmg", this is the installation file. You can copy this file to your desktop and restore it onto a USB drive using "Disk Utility". This creates a bootable installation for 10.8 that you can boot into and use to reinstall.