1. Force the System to Shut Down and Reboot
A manual shutdown will allow you to unlock and reboot the system. Pressing the power button may not work unless it's as instructed below. As a last attempt you can try pressing the <Ctrl> + <Alt> + <Delete> keys at the same time to see if the Task Manager appears. You can also try waiting several minutes to see if the system resumes. If this has been attempted but the system is still locked with nothing responding, perform the following steps:
NOTICE: Any data that was not saved before the system locked up will be lost.
Press and hold the power button for 8 to 10 seconds or until the system fully powers off.
Press the power button as normal to turn the computer back on.
Sounds like instructions from Dell for solving your problem?
Unplug the ac adapter and take out the battery for a few seconds, to fully shutdown the laptop. Then try the above instructions again.