I have a Canon PowerShot SD790 IS. The camera

The error message indicates an error that involves the lens unit or lens cover.
This error may appear where the lens is prevented from properly extending or retracting such as, for example, when pressure is applied to the lens or the area surrounding the lens while handling the camera. The error may also be displayed if the lens unit/camera has been subjected to an external shock, or if liquid, sand or dirt enters the camera.
This issue may also happens if the Battery is not fully charged.

In this case, charge the battery completely then try to turn on the camera. Furthermore, try resetting the Camera to the original factory settings. You may also try hitting the lens gently against your palm after turning it ON.

If the issue persists then service is required.