No, I not sure that's it. As I said
I find this surprising but I think you need someone with more experience than me for further analysis.
and I bet somebody has been through this before.
What I understand is:
1. The lights are on. This suggests power is getting through and the electronic components most likely to fail haven't.
2. The PC cannot detect the Arduino, no d-ding. This could be due to a failure at the Arduino USB port which indeed was
caused by a power problem, but the power supply itself has survived.
I guess the second only makes sense if the damage was done when you tried to power everything through the USB, but you did have some results after you went to the 18v supply.
In the lack of better advice, I would closely examine the USB port, sight and smell. You should be able to test for continuity. If it is a USB "B" terminal, you might consider replacing it. That should be cheap and easy, but I think a mini port would be a very different proposition. You might even just wire an old USB cable to the board, bypassing the port.
Nothing vetured = nothing gained.......