Gray stopsign on startup?

[l Panda Bear l]

I have an iMac and decided to update my softwares.
It told me to restart, and once I did I saw it installing.
After, my cousin accidently turned off my computer and now when I try to turn it on everything is normal until the apple logo changes into a gray stop sign.
Can anybody help me?

 

[BlueRevolution]

Your update was botched. Insert your computer's restore disk and boot to disk by holding C at startup. Set the installer to archive & install, and select preserve users and network settings. When you're back up and running, run Software Update a few times and you'll be good to go.