MacBook Air shows a circle with a line thru during boot up after I try to update to El Captain

After I download and install El Captain it booted up, but got stuck so I turned off and when it tries to boot up it start with about 3/4 of the bar full then shows a circle with a line through it. Can someone please give me some things to try to fix. I looked around I guess it could be my hard drive. I don't need anything that is on the computer so a complete reboot is fine with me.