strangest thing ! any one ever experience this?

okay the compputer has to front panel light. when i turn the computer on the green light is steady telling me that the sys is on and the HDD light is blinking red to edicated activity in the hard drive.. here is the issue .. it started 2 days ago and it does not happen all the times .. ( ie every time i turn the computer off) i turn the computer off and it go through the process of shutting down and evry thing turn off except the green light and i can hear the fans running .. no HDD light and since the computer is tenchnically off so when i press and hold the power button to do a hard reset nothing really happen cuz the HD is really off.. but like i said it does not happen all the time .. when it happen i had no choice but to un plug from the wall then i turn the computer back on and try to shut down again , it will shut normal