one diode in any of the LED backlight strips can cause the power supply to shut down. There are 14 strips in that unit and the cost of new strips is about 60 dollars. They are not easy to find, you can try testing each LED by hand and locate the bad one.
If your Hitachi is not Amazon fire TV based or Android TV based then there probably isn't a huge app store. For $39 get an Amazon fire TV stick it will solve your app issue
Try unplugging it and wait a bit before plugging it in. Plug directly to wall instead of surge protector. If all this fails it may be toast as was mine but contact Hitachi before recycling it.
You can change input using inputs button on remote or the TV button to bring it back to the correct live TV source. Here is your manual: Hitachi 51F59 Operating Manual