Caps Lock key blinks error, device does not boot

I have been helping my brother troubleshoot his laptop, which seemingly died after connecting a device to the USB port. He told me that he connected a high-powered USB device to the laptop, and the display instantly went black.

Holding the power button to power on the computer results in only the fans spinning up. The display does not light up at all. After the computer turns on, the LED light on the Caps Lock key begins blinking a 3.5 code, which appears to mean “The CPU is not detected.”

I have already tried the following troubleshooting steps:

• Unplug and reconnect the battery
• Power on the computer with the internal keyboard and trackpad cables disconnected
• Unplug the CMOS battery for a few minutes
• Power on the computer with the power adapter connected, and the battery disconnected.

I would appreciate any advice on further troubleshooting or repair that I can do to attempt to get this laptop working.