I think I blew up my Mega 2560


I can no longer upload programs to my Arduino Mega 2560. I’ve tried a different serial port and a different computer and it tells me it’s timing out:
avrdude: stk500v2_ReceiveMessage(): timeout
avrdude: stk500v2_getsync(): timeout communicating with programmer
An error occurred while uploading the sketch

I’ve ordered a new board. My question is I don’t know how it happened. I had the board attached to a USB port on my laptop. The board is wired as part of an old electro-mechanical pinball machine. I was running tests and connecting wires when all of a sudden, I turned on the pinball machine and my laptop shut down. I have no idea how that happened, but after that I couldn’t upload sketches anymore. When plugged-in to the USB, the board still shows the green light and a solid yellow light on “L”.

If anyone has any ideas what could have happened please let me know so I can avoid blowing up any more boards. Thanks.

That is shut down the computer as well strongly suggests you short circuited it.
Now doing that with 18 volts is almost certain to cause untold damage.

If you have an ICSP programmer you could see if it was just the USB side that let out the magic smoke.
If not put it down as a learning experience to double check your wires in future before you power up the board.