Serial port not available on Arduino IDE

While connecting the solenoid valve circuit (as shown in attachment), I needed external voltage from the wall socket. I used a 15v 550mA adapter. After uploading the code and plugging in the adapter to the barrel jack, the Arduino did not show up in the serial port of the ide and we couldn’t connect to it anymore or upload anything thereafter.

Has the board been overloaded? Although for the Arduino Mega 2560, the input voltage is shown as 6-20V.

Any fixes would be appreciated

Board: Arduino Mega 2560.

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