Is your firmware (Marlin?) installed on the Mega and is it responding to commands from PC or LCD controller? Did you configure and upload the firmware yourself?
Is the problem just with the hotend? Can you control all the other components (axes, heatbed) normally?
What is the resistance between "-" D10 and GND? (measure when not connected to power!) If it is ~0 you might have a short of a bad MOSFET.