The reason you have gotten no responses is because you posted a question with multiple answers here on our thread - and thousands of answers on google - In the future try providing some more information, like what exactly you have plugged in. You could do a MRoogle/google and find out exactly how to connect your iMac to your HDTV.
Make sure your TV is on RGB/VGA/Computer - whatever input coincides with the VGA input. Make sure everything is properly connected - and under displays in System Preferences there are 2 displays listed so you can rule out any issues on the computer side.