First my Parallels froze and as part of my attempt to fix that problem I was rebooting my iMAC. Now it will not boot. Get a diagnostic message" -3403D" when I try an Internet start.
As above
iMac 27″, macOS 10.13
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Do you have a full and current backup of your hard drive?
Can you boot into the Recovery volume (boot with the Command + R keys held down)? If you can launch Disk Utility and run First Aid on the drive. Then try a normal boot.
If the normal boot fails boot again into Recovery volume and reinstall the system. If that fails and you have a current backup of your boot drive go back to the Recovery volume, erase the hard drive and reinstall the system. Then use Migration Assistant to get your login account and files onto the newly created volume.
