Prohibitary sign at boot up- help??

hey,so turned on laptop , began loading usual, tone, apple logo spinnig wheel, showed grey prohibitary sign?

i've read few posts far havent found solution has worked

- far aware not hd problem under disk utilities says hd seems ok no problems.

ideas can fix this? im pretty new mac user @ appreciated!!

 

have tried of these steps?

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​

, solutions did try, unsuccessful, don't keep guessing , repeating.

 

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