My imac is dead, push the start button, nothing happens. Screen remains dark.

Please use If your Mac won't turn on or start up , if it still won't start after following the troubleshooting steps, then you need to take it to your local Apple Authorized Service Provider to be serviced.

Unplug the power cord from the back of the Mac for about 15 seconds, plug it back in and try the power button again. If it does boot up you probably have a failing power supply.

Let us know what the results are.

I've already instructed you on the next step, please contact your local AASP to have the computer serviced.

Your welcome and good luck.