The simplest fix is to wait until the battery runs out. The sensor should work correctly after recharging.
If the green LED doesn't turn off with your sensor, there are two things you can try to get the sensor to work correctly again.
Plug your sensor into a computer
Simply plugging the sensor into a computer can reset the device. There is no need to connect it to Protern.io.
Letting the battery run out
If plugging the sensor into a USB port doesn't work, then you will need to let the battery run. Once the sensor has completely died, which can take 6 hours, you will need to recharge it. Afterwards it should work fine.