sketch doesn't start by powering the board with an ethernet shield on power

I’m new in arduiono. studied the blogs and documentation. I have a arduino Mega2560 Rev3. I first loaded a simple sketch to the arduino without a shield. The sketch works fine. Then I connectetd an ethernet shield. I loaded a sketch that is initializing the ethernet shield and works with it. After loading the programm it works fine.Then I disconnected the usb from arduino and connected it to an external power supply. The sketch doesn’t work. The ethernet shield will not be initialized.

I also tried to connect RX0 to TX0 pin, but it doesn’t solve the problem.

I don’t know, how to solve the problem. Pleas advice

Thanks
hacki

What external power supply are you using? What is the volt/amp rating on it? DC or AC?

Is it an Ethernet shield with PoE support?

Hello, the ethernet shield has no PoE support.

I used the USB Power with 5V= and 750 mA. Isn't it usable ?

Thanks Hacki