iMac shows prohibitory symbol
This happen to me too after I started and then canceled the installation of Mojave.

I fixed it easily by following these steps.


Boot in recovery by Powering up and immediately holding CMD+R
Clicked on Apple logo and choose Startup Disk
Select the Mac OS disk and click restart.


If these steps didn't work then you can try reset. 
WARNING this will delete all your Files


Select Disk util from the recovery menu
Erase Disk
Go back and Reinstall OS