I'm not aware of one long beep meaning anything specific but I have encountered this myself and the problem seems to have been due to the graphics card not being recognised properly during the POST phase. I'm afraid I can't be more specific than that as I still haven't got to the bottom of the problem and my graphics card is on it's way to Taiwan for Gigabyte to run some further compatibility tests.
There have been instances though, where one long beep has been due to RAM issues, which only confuses the problem further which is why tazman_75's advice about trying to reseat things is good.
If everything has been working fine then the only other cause could be a hardware update or change to the configuration of your HDD's and any add-on cards which is why I asked if you have made any recent changes to your system, such as a BIOS update, new HDD, SSD or DVD/CD Burner, something like that.