Which arduino board is this with?
Can you describe the behavior you're seeing more precisely? "Once an external power source is connected the serial monitor stops working" is missing a bunch of the key pieces of information we would need in order to point you to the next step of solving the problem.
Did you have a serial connection open at the point in time when you connnected an external regulated 5v supply to the 5v pin of the nano, and this open connection ceased to function? In this case, if you tell the software to close/open connection, does it work?