Try the following steps in order:

1. Check battery: Ensure the controller is charged. Plug in via USB to charge if needed.
2. Check the mode: Make sure the correct wireless mode is active — LED 1/2/3 for Bluetooth, LED 4 for 2.4 GHz.
3. Check for existing connections: Ensure the controller is not already connected to another device. Turn off Bluetooth on other paired devices.
4. Re-enter pairing mode: Hold Pair for 3 seconds until the LED flashes rapidly, then search for the controller in your device's Bluetooth settings.