I can’t start my computer after installing update

Question

I have an 2010 iMac running High Sierra. Recently I got a notice that there was an update to my system. I’ve installed a few and they worked fine. I installed this latest one tonight but my iMac will not restart using this new upgrade. When I try to start my computer it loads a small part of the system and then just sits there with no advancing of the slider. I have tried starting in safe mode but no luck. I tried restarting in recovery mode but my recovery disk is apparently not working.

Is there any way to start my computer? I am leaving it in the startup mode all night to see if it will eventually load but I’m not very optimistic. Is there another way to get it to startup? I thought I had a backup disk but that method doesn’t seem to work. I am completely out of ideas and my computer is a brick. Any help would be greatly appreciated.

Answer 1

Start from Recovery & re-install the OS.

This will lay a fresh OS over the old one without affecting anything else on the Mac.

I've had this happen several times on different Macs with High Sierra security updates.

Answer 2

Well my attempt at installing the System update worked after all. I started it installing and went to bed. When I got up this morning the update had installed and seems to not cause any problems - yet! I think maybe I was too impatient. But when I tried it I waited for over 30 minutes and the slider had not moved. I don't know how long it took but at least I got it installed. So lesson learned is if your update looks like it won't install, let it run all night and see if that works.