Hi,
The overvoltage probably damaged components on the power supply board in the keyboard.
With the power totally disconnected from the keyboard open it up and locate the power board (where the power cord socket connects to) and look for any burnt out or blown components.
If you are unsure if anything is wrong post some close up images of the board back here and hopefully someone may spot the problem.
Here’s how to do this.
[guide|21499]
Do you have a DMM (digital multimeter) and know how to use and also do you have soldering skills and tools it as they may come in handy when trying to repair the keyboard?
If not then contact a reputable, professional electronics repair service and ask for a quote to repair the keyboard.