Wireless earbuds or headphones may free you from the tangle of wires, but they offer another annoying problem. Sometimes third-party Bluetooth devices have trouble connecting to your iOS device after you remove or shut them down. Here's a few possible solutions to connect your wireless earbuds or headphones to your iPhone.
Navigate to Settings > Bluetooth to check if your device is connected to your iPhone. If it says "Not Connected" next to the name of the device then you must try and connect that device. Make sure that the device you're trying to connect is on and in range of your iPhone, then tap the name of the device in the "My Devices" section to try and connect.
If you device will not connect, tap the "i" in a circle icon, then tap "Forget This Device." Tap the name of the device when is reappears to try and connect.You can also tap the Bluetooth slider to disable Bluetooth on your iPhone, then power down the device you're trying to connect for a couple of minutes. Power on your device, then tap the Bluetooth slider so it is ON/GREEN. Tap the name of the device in the "My Devices" section to try and reconnect it.
Sometimes returning your earbuds to their charging case will also help reset them so you can reconnect them to your iOS device.