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
Do you have a WiFi dongle connected to the TV?
If so, it would appear it has a bad connection and needs to be reseated (unplug->plug back in).
If that does not fix the issue, the dongle may be faulty.
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.
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.