There is a 12 month manufacturer’s warranty on the P-115 digital piano.
Given that the digital piano is only 6 months old, I suggest that you first verify the purchase date of the digital piano to ensure that the warranty period is still valid. If it is still valid, consult the manufacturer’s warranty statement that came with the piano, as to what you have to do to make a claim for a manufacturer’s warranty repair or replacement.
Here is a link to the Yamaha USA warranty for the piano. A quick check online shows that the warranty period for this series piano is mostly the same worldwide.
If you can’t find your warranty statement, search online for Yamaha P-115 warranty for your location
I cannot find a service manual for your model digital piano, however I have found one for a Yamaha P-120 which I hope may be close enough to your model to show you how to gain access the the unit’s On/Off switch so that you can test it with a Ohmmeter (with the power disconnected) to see if it is operating/releasing correctly.
Besides testing just the switch, also check that there is no short circuit (s/c- bit of loose wire etc) between the wires on the connectors as shown by the green arrows in the image below. The red arrow shows the switch in question. Do not be confused by the switch shown by the blue arrow as this is just an overload current protection switch and not the On/Of switch.
As I said earlier, I hope that your model will be similar to this one, at least as far as this part of the circuit goes.
(click on image to enlarge for better viewing)
Here’s the link to the service manual.
When the sentence below the Document preview box changes from This file is downloadable free of charge: ...processing... to This file is downloadable free of charge: Download , click on Download, as this is the download link. Be patient, it takes a little while to find the file.
According to the circuit, the switch (SW201) has a parts designation of VQ670600. This doesn’t mean that your switch is the same or has the same part number. I have only included this information hoping that there may be a part number stamped on your switch and it will be the same or very similar, (given the closeness of the piano model numbers, I’m hoping that they may have used the same parts for a lot of the components).
It may be that if the switch is faulty , (searching online, using this number gives no results), that you may be able to contact Yamaha spare parts for your location and quote the part number, found on your switch, to them (if there is one), to see if they can supply just the switch.
Also just reminding you that if you decide to open the piano to try and locate the fault that you’ll be voiding the manufacturer’s warranty.