Samsung Galaxy Mega i9205 Soft reset

Press and hold for more than 7 seconds to reset the device if it has fatal errors or hang-ups, or freezes.

Your device freezes or has fatal errors
If your device freezes or hangs, you may need to close programs or reset the device to regain
functionality. If your device is frozen and unresponsive, press and hold the Power button for
more than 7 seconds. The device reboots automatically.

If this does not solve the problem, perform a factory data reset.
use menu option in settings 

On the Applications screen,
tap Settings - Accounts - Backup and reset - Factory data reset - Reset device - Erase everything.

gt i9205 Factory data reset: Reset settings to the factory default values and delete all data.

Encrypt external SD card: Set the device to encrypt files on a memory card.
If you reset your device to the factory defaults with this setting enabled, the device will not be able to read your encrypted files. Disable this setting before resetting the
device.

or

Use this method to hard reset your Samsung Galaxy Mega GT-i9205 Android phone: Enter *2767*3855#. Warning! All your data including contacts, messages etc. will be lost! Copy all your necessary data/Contacts/Messages etc. to SIM or make backup to your PC before full reset! Take out SIM card before full reset.