Your smartphone may be dead for a variety of reasons. It could be a short in your internal circuit or on the PCB in your smartphone. Capacitors are frequently short-circuited. If your smartphone comes into touch with water, your capacitors are likely to be short-circuited. Another explanation could be a problem with your smartphone's Power IC.
If the above two explanations aren't the issue, look into other connectors such as the battery connector, resistors, inductors, display connection strip, and other connector strips. Lockage in PCB components is another possibility. To begin, retrieve the linkage with a hot air gun. For example, if the power IC appears to be licking, use a hot air gun to dry it (make sure you are removing the power IC at the first stage). If the problem persists, the Power IC should be replaced. If any capacitors are found to be faulty or damaged, they should be removed.