Normally, you can easily reconnect your headset by switching on
Bluetooth on
HTC Desire 520, and then turning on the headset.
However, you might have to connect manually if your headset has been used with another
Bluetooth device.
If you still cannot reconnect to the headset, follow the instructions in
Unpairing from a
Bluetooth device, and then follow the steps under
Connecting a
Bluetooth headset.