It usually happens if our Bluetooth is turned off or it is not in pairing mode so go to setting and turned on Bluetooth. even if our Bluetooth is turned on, our phone might not be in discoverable mode .so bring your device near to Bluetooth and search for available devices you can see your phone tap on the phone, and allow pairing your device gets connected to Bluetooth if your device is already paired and since not connecting to Bluetooth then forgot device and again pair again with Bluetooth your problem gets solved.