What does "X" in the battery indicator mean?

[Windows Central Question]

My Surface Pro (Windows 10) has suddenly developed a problem of not running unless the power cord is plugged in. Pressing the power button does nothing unless the device is corded. If I pull the cord while the device is running it immediately goes into sleep mode as if I had pressed the power button. The battery indicator in the system tray displays a white "X" rather than a battery level. Clicking the indicator shows the battery is charged to 100%. See this screenshot: bit.ly slash 1Uh2l3b (sorry, I couldn't insert a clickable link). Does anyone know what is causing this problem?

 

[Maurizio Troso]

My Surface Pro (Windows 10) has suddenly developed a problem of not running unless the power cord is plugged in. Pressing the power button does nothing unless the device is corded. If I pull the cord while the device is running it immediately goes into sleep mode as if I had pressed the power button. The battery indicator in the system tray displays a white "X" rather than a battery level. Clicking the indicator shows the battery is charged to 100%. See this screenshot: bit.ly slash 1Uh2l3b (sorry, I couldn't insert a clickable link). Does anyone know what is causing this problem?

Battery not plugged / battery fault