Power loss during update or wrong firmware file. Fix: Place earbuds in the case, connect the case to a charger for 2 hours. Then try a hard reset (press and hold the case button for 20 seconds). If that fails, contact QCY support with your proof of purchase – some models have a factory restore via service center.
This is where the becomes critical. Did your earbuds suddenly stop pairing? Does one side keep disconnecting? Is the battery draining faster than it used to? A simple firmware update might fix it all. Qcy Firmware Update