Try turning off the phone by holding down the power button for an extended period of time. If this doesn't work also try plugging the phone in as well as removing the battery pack to force a restart.
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For more information reference this [https://www.ifixit.com/Wiki/Samsung_Galaxy_Mega_Troubleshooting|troubleshooting guide.]
Try turning off the phone by holding down the power button for an extended period of time. If this doesn't work also try plugging the phone in as well as removing the battery pack to force a restart.