Hi, my 21 inch Mac is not turning on.

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

Try this: remove the power cord from the back of the iMac for 10-15 seconds. Plug it back in and try powering it up again. If that works but it shuts down my itself or won't boot up without pulling the plug again then you have a failing power supply. It will have to be replaced by an Apple Genius Bar or Apple Authorized Service Provider.  

Welcome to the Apple Support Communities.

See: If your Mac doesn't turn on - Apple Support.

Jack