White square on the screen

Ouch ouch ouch

2.
3.
Tested?

I don't really believe it, but I had forgotten this manipulation before cooling/resting the battery:

To close/stop all ongoing tasks/applications, you need to press and hold the Sleep/Power button for at least 10 seconds (normally it's the same with iOS).

Hello,
Take a backup and reset it

--
The stone and the pebble fall at the same speed (an ancient Chinese proverb)

But I suppose you have already tried restarting the smartphone...
If that doesn't work...
This trick is much less known, if it's a model where you can remove the battery (let the battery rest for 15 minutes outside the phone, that is).... If the battery is trapped, then turn it off for a long time, like 1 hour....

The phone is like when you turned it on for the first time with the square!

A simple thing, try restarting it 5 times in a row since it worked a bit with a restart

Last resort:
If you care about your phone,
Find out how much budget you would need for a repair
There are many repair shops in Paris (from simple to double)
But you need to live in the city or a big city.

Re I'm back ;) I can't believe it, but if you want to try, apparently we are persistent, so I always say "nothing ventured, nothing gained," a very well-known saying ;)

It's about a problem C) G)
G) for my classification it's "won't start/black screen"
C) for my classification it's "display issue," but since it's not continuous... it's not really a C) and it's a problem let's say original LOL

C) G) Plug the device into a power source for at least 5 minutes, then simultaneously press the Power/Lock button and the bottom of the volume button ("volume -"), then release the Power/Lock button. Do the same if nothing happens with ("volume +")
[For models with a central general button located at the bottom of the screen, it will always be the case with iOS, do it with this one and the Power/Lock button if it doesn't work. Start again with ("volume -") and then it will leave ("volume +")]

C) G) [Like above with a computer if you have one]
Plug the device into a computer for at least 5 minutes, then simultaneously press the Power/Lock button and the bottom of the volume button ("volume -"), then release the Power/Lock button. Do the same if nothing happens with ("volume +")
[For models with a central general button located at the bottom of the screen, it will always be the case with iOS, do it with this one and the Power/Lock button if it doesn't work. Start again with ("volume -") and then it will leave ("volume +")]

C) G) Plug the device into a power outlet for a few seconds, then hold down for 5 minutes (it's not an exaggeration, really long, which will allow the phone to get a little charge and restart on its own!
=> What I had already mentioned but in +++