Q: iMac showing prohibitary sign.

HI Dasling, see these Apple Support Topics:

If your Mac won't turn on - Apple Support

How to start up your Mac in single-user or verbose mode - Apple Support

Reset the SMC then startup in Safe Mode

Reset the System Management Controller (SMC) on your Mac - Apple Support

Run Disk Utility > First Aid and tell us what it reports

Start up in Safe Mode and see if the problem still occurs.

Restart holding the "shift" key.

(Expect it to take longer to start this way because it runs a directory check first. Also give it a little time after start because it runs some maintenance routines in the beginning.)

Startup in Safe Mode

https://support.apple.com/en-us/HT201573

At the prompt copy and paste use fsck fy

Press return key and until it returns to the prompt

Reinstall OS X

(you will need an exteranl disk and try these steps again.

If these steps don't work take it to any Apple Store for a free diagnosis.

If you have tried EVERYTHING, then there is nothing one can do to assist you.

Was a part of everything, SMC, NVRAM resets and the use of an external boot drive?

Ciao.