Device shows " no command" on screen & won't turn off after turned on
The screen showed an Android robot logo lying down with the chest cover open & exposing a keypad. An exclamation mark framed in a red triangle was shown beside. A foot note below the logo says “ no command”. I pressed the on/off button to try to reboot and the logo dimmed with another note: ‘Now send the package you want to apply to the device with “add sideload <filename>”…’ shown at the bottom of the screen.
I didn't know what that note meant. I pressed the button again and that note disappeared with the robot logo turned brighter as before. I pressed more and it just repeated the scene again and again. I got stuck and cannot even turn off the device. Could this be an Android operation system problem and fixable? Thanks