I have an Android device which had a custom os..It was for educational purposes and I could not even open another app on it. After the end of the subscription period I thought of using the device. I tried opening it's recovery mode but instead opened some factory or diagnostic mode which was in Chinese. By mistake I pressed the power key to select an option ending with "emmc" and now It boots but shows a black screen with navigation buttons and time...Any keys pressed make no effect and I am unable to shut it down. When I press the power key for a few seconds it reboots but shows the same screen. I just want to shut it down somehow. Please Help!!
Edit: I found a way to shut my phone down. I connect it to my pc and then press the power button for 10 seconds then the screen goes off and the charging percentage shows up after a couple of seconds, I pull the usb cable out and the screen disappears and voila ! My phone is switched off.
Thanks for all your response.