I left my IMac doing a virus scan overnight, now I have a black screen and ball of death. Can't log in. I've recently upgraded to Yosemite and the Mac is 5 years old

What application did you use for the virus scan?

Macs don't need a virus scan.

PeachesFinbow wrote:

I left my IMac doing a virus scan overnight, now I have a black screen and ball of death. Can't log in. I've recently upgraded to Yosemite and the Mac is 5 years old

Try starting up in safe mode:

OS X: What is Safe Boot, Safe Mode? - Apple Support

If you're able to start up that way, some third-party software is likely to be the culprit. What anti-virus software are you using? Depending on the answer, that software may very well be the cause of the problem.

If you're unable to start up in safe mode, try repairing the hard drive:

http://pondini.org/OSX/DU6.html

If the repair is unsuccessful, your hard drive is probably dying and will need to be replaced. If your hard drive doesn't even appear as an option to repair, it's already dead.

Press and hold the power button for a few seconds until the power shuts off, then release, wait a few more seconds, and press it again briefly to restart.

If you can log in now, back up all data, then get rid of your worthless "anti-virus" software according to the developer's instructions.