Keyboard reconnect?

My Bluetooth keyboard ran out of battery power. I replace the 2 AA's, but now I can reconnect the keyboard. It has disappeared from the list of devices and. none of the Bluetooth Assistants seem to be able top find find it. I think it's turned on/discoverable (1 short push), but the green light doesn't blink after it comes on. I have the sinking feeling that I'm missing something mindboggling simple, but can't put my hand on it. It be very grateful for any pointers.