after downloading the update of new osx, macbook failed to restart

I downloaded the new OSX Siera, and left my macbook air to install.

when I came back, i found folder icon blinking with a question mark on it, and could do nothing but force shutdown and restart and go back to the same issue.

Please help.

How can I either restart or recover to my old osx siera?

Note that I didn't take any backup, and while downloading the new update from the app store, it was interrupted several times, and I restarted the donwload.

thanks to help with this.