Why Arduino doesn't start automatically after power cut

I have an Arduino mega and w5100 Ethernet shield connected to Internet.

Whenever there's a power cut or I manually unplug and plug the power source for Arduino it doesn't start automatically. It starts again when i press Reset button on Arduino.

Whats's the problem here?
How can i fix this?

My Mega and W5100 shield restarts fine after a power cut.

The problem could be in your code (which you didn't post), or in the hardware (post a diagram and/or pictures).
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