keyboard will not connect

My Bluetooth keyboard will not connect to my iMac. It is a couple of yeas old and was working fine when I went to work. When I came home it wasn't.

The batteries are freshly recharged and the green LED comes on for a few seconds when I press the power button. The Apple magic mouse is working fine. Any ideas.

NEVER MIND Got an iPad a while ago and the Bluetooth was on and it paired with my keyboard.