I have an idea. First, check the battery. Typically, a battery lasts around 3 to 4 years, so it might have lost its ability to power the device or could be completely dried out. Try boosting the battery and then charge it for 1 to 2 hours. Check if you're still facing the same issue.
If the issue persists, go to a spare parts store and check the specific battery model number. Test it there itself—if the phone turns on, it’s time to replace the battery. If the problem continues, check the charging port using a digital multimeter.
If the charging port is working fine, the issue might be with the display. Try using a spare display to see if that resolves it. If the problem still remains, it’s likely a CPU issue. In that case, you may need to reball the CPU or replace the motherboard.