I don't understand exactly what is happening to your system. Do you mean that the system has hung up and you can't do anything? Does the mouse cursor still move? What is displayed on the screen?
Have you tried force quitting applications that may be hung? Here's an article that will tell you the different options for force quitting applications:
If you want to try and restart the iMac you can just hold the power button down for ~10 seconds. If you only hold the button for a couple of seconds the display will go black, but only because the system went to sleep. To force shut down the iMac keep the button held down for approximately 10 seconds until you hear the fans and hard drive shut down. Once it shuts down, press the power button again to see if everything boots normally. If it does, run the Disk Utility application located in the Applications/Utilities folder so that your can verify the disk and repair permissions.
If you continue having problems plear provide more information including which version of OS X you are running, what you mean by 'standing still', and whether you recently installed new software or updates that may be related to the onset of your issue.