My iPhone won't turn on after the battery drained totally

Theres two things that are common in this situation

1. The battery protection circuit was triggered and the battery was disconnected from the phone to prevent over discharge. you can verify if this is the case by measuring the battery with a multi meter and seeing a voltage present (usually its 3 or more volts)
2. The tristar chip has been damaged and will not charge the battery when it gets too low. if the battery displays a voltage on a meter this is the most likely cause and requires micro soldering to repair.

Small note; it is possible for residual power to be drawn from the phone and cause the battery to drop below safe levels even when off, triggering the over discharge protection. This is much harder to test for and requires powering the phone while measuring current draw.