The power button of my K1 tablet suddenly seemed to stop working. The only way to get the screen back on is to plug in the charger to the tablet.
Try charging the tablet fully overnight, then try restarting from battery. Note that if the system is fully shut down, you must press and hold the power button for several seconds to start the boot process. The two white LEDs on the right side of the tablet will flash as once as the system starts.
If charging it overnight does not help, you may attempt a hard reset by pressing the power button while holding both volume buttons.
If the tablet indicates full charge and will not power on other than when connected with charger input, then contact the nearest Lenovo Service Center or call technical support to continue problem determination and arrange service.