Since the PSR-E443 was released 4 years ago, I'm guessing your keyboard is no longer under warranty, unless you just recently bought it new?
The best thing to do would be to take it to a certified Yamaha service technician for repair.
If that's not possible, the service manual is available at ElektroTanya:
https://elektrotanya.com/showresult?what=Yamaha PSR E443&kategoria=&kat2=all
It sounds like the "Standby/On" switch isn't working. The part number is listed as WZ373900.
If it's the circuit board that the switch connects to, that is part number ZH796700.
There are merchants who sell keyboard replacement parts online, including Yamaha24x7.com-- but I think Yamaha24x7.com services the USA exclusively. I did search their site and those two parts are available.