HELP: tried to update software, now my iMac won't start

i ran software update gave me error , asked me restart. hit "ok" , when computer tries boot, gets load screen in addition normal "spinner" has loading progress bar. every time gets 25% through progress bar , shuts off.

tried zapping pram , have tried starting safe mode neither work.

 

here other options try:

mac doesn't boot

resolve startup issues , perform disk maintenance disk utility , fsck

troubleshooting: computer won't turn on

a flashing question mark appears when start mac​

 

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