First, verify that you are within Bluetooth range (~10 feet, 3.5 meters for HearLink; ~200 feet, 60 meters for HearLink PLUS).
If that is not the issue, and a long time has passed between uses, or if the smartphone has been playing an audio stream, the headset may not automatically reconnect with the HearLink or HearLink PLUS transmitter when you are within range. In this case, a short press on the headset's Power button should re-establish the connection with the transmitter.