Hello. You will need to do some basic diagnosing. Firstly, measure the output voltage of the Power supply. You will need a basic multimeter which you can get for under twenty bucks. Assuming the power supply has the correct voltage, you will need to split the keyboard and have a look at the power board. Most Yamahas have an in line fuse which is a square device about a cm high. Measure it for continuity. Measure the input is not shorted as they usually also have a DC mov. What is very common on Yamaha keyboards is the w9825g RAM chips on the main board failing. With power plugged in, touch the chips and see if one or more are hot. Replacing them is very tricky if you are new to soldering. Another common problem with Yamahas that have been overvoltaged is the y164 amp chips on the main board. They are between the four chokes and they will be quite burnt and the pcb might be damaged as well. They are cheap but again, best left to someone with good soldering skills
Thank you
Chris Kupke