I can’t start my computer after installing update 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.
Start from Recovery &amp; 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.