If you have connected power outside what is specified in the DK HW user guide then indeed you may have damaged the board. Before determining that I would check with multiple different USB cables on different PCs to make sure that is not the issue.
There is a LED close to the debugger, LD1, what is this doing when you plug in the DK?
If it blinks rapidly then you have a problem with the USB cable or the connection to the PC. If it blinks and then goes stable on then they USB is working and then it could be an issue with the drivers on the PC. If it doesn't light up at all then you are more likely to have fried the kit.
LD1 did not light up, but the other 54L15DK is functioning normally, so the computer is working fine.
When I connect VDDIO to 3V, LD1 will flash a few times and then go out, sometimes it will continue to flash, and the computer can recognize the J-Link.
Applying voltage on random pins on the board is a sure way to burn it so I would not be surprised if this has been damaged if that is what you have been doing.
Default voltage on the board is 1.8V so seeing 1.74 is inside expectation. Applying 3V from the outside when the board is set up for 1.8V is not OK.
