My HP Prime G2 doesn't turn on anymore

[RONana]

Hello everyone!
After a 2 month break from college I tried to turn on my HP Prime but it looks dead. The display remains black. Without battery, powering the calculator via USB cable, I see the display backlight but nothing more (the display remains black).
The connectivity kit does not detect the calculator.
I checked the battery voltage (~ 3.7V) and the voltage from the calculator pin via USB (~ 4.2V). I tried to reset the calculator using the reset button on the back, but nothing happened.
I left it on charge overnight and with no battery overnight, but nothing has changed.
Do you know how I can solve this problem?
Thanks in advance!