Why did my phone suddenly turn off?

@muhammadfa17935 first try to do a restart use the instructions from here If that does not yield any results you may have some power issues. For that, I would suggest you replace the battery as well as the sub-board. Double check the flex cables going form the charging board to the main board. Make sure it is properly connected, no tears and no corrosion.

Anything after that will most likely be related to the circuitry on your main circuit board. For that you will need special tools and skills to test it. Of course, you will also need the schematics and component layout for your y20

Since iFixit does not yet have specific guides for this, use something like this video Batteries and sub_board are readily available at many online stores.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!