System update crashed my Mac

So about a month ago, I turned the system update on and went out. Few hours later I was back and the computer was off. Tryed many times, but it doesn't start up. Update screen shows up (Apple icon and a sliding strap), but when it shows 100% done, the computer powers off. What shall I do to avoid clearing my HDD drive?

iMac 21" 2009, not sure what system it was, couse I didn't use it a lot, but definitely not up to date 😉