My phone got blank while using, now its not turning on!

I'm sorry to hear about your POCO X3 Pro. This is a known and frustrating issue with this particular model, often referred to as the "dead boot" problem. The fact that it turned off suddenly while you were using an app, and now won't turn on even after charging and changing the display, points to a deeper hardware issue.

Here's a breakdown of the problem and the potential solutions:

The most common reason for the POCO X3 Pro's sudden death is a faulty connection between the CPU (System on a Chip) and the motherboard. Over time, due to heating and cooling cycles, the solder joints holding the CPU can become weak or crack, causing the phone to fail. This is a widespread manufacturing defect with this model.

Since you've already tried basic troubleshooting like charging and even a display change, the problem is almost certainly not the battery or the screen.

1. Professional Repair (Reballing):
   • This is the most effective and common solution for the dead boot problem.
   • A skilled technician will carefully remove the CPU and RAM from the motherboard, clean the old solder, and "reball" them (apply new, high-quality solder balls) before resoldering them back onto the board.¹

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• This process effectively re-establishes the connection and brings the phone back to life.
  • What you should do: Find a reputable repair shop that specializes in motherboard-level repairs. Ask them specifically if they have experience with the POCO X3 Pro dead boot issue and "CPU reballing."
• Warranty Service:
  • If your phone is still under warranty, this is your best option. POCO has extended the warranty for the X3 Pro in some regions due to this known issue.
  • Contact a POCO or Xiaomi authorized service center. They should be able to replace the motherboard or repair it for free under warranty.
• Advanced Troubleshooting (Requires technical skill):
  • Some users have had temporary success by applying heat to the motherboard, a process sometimes called "baking" or using a heat gun. However, I strongly advise against this. It's extremely risky, can damage other components, and is often only a temporary fix. It's not a reliable solution and can make a proper repair impossible.

• Charging: Since the motherboard is the issue, it can't manage the charging process or power the device, so charging won't help.
• Resetting/Booting: The phone can't get past the hardware failure to even begin the boot process. Holding down power and volume buttons won't work if the core components aren't functioning.
• Changing the Display: The display is just an output device. If the CPU and motherboard aren't working, they can't send any signal to the screen.

In conclusion, given the nature of the POCO X3 Pro's dead boot problem and the troubleshooting steps you've already taken, the most realistic and lasting solution is to take it to a professional for a CPU reballing or to a service center if it's still under warranty.