Ensure the phone is connected to a working charger and leave it for at least an hour. Try different charging cables and adapters.
Check for visible physical or water damage that could affect functionality.
If the battery is removable, take it out, wait a few minutes, and then reinsert it.
With the phone off, press and hold the Volume Up, Home, and Power buttons together to see if you can enter Recovery Mode.
If the above steps don’t work, it’s likely a hardware issue.