My cellphone is completely dead and won't turn on.

@arulselatan TRy a hard reset by fiollowing the insructions on here. Or you can try to get in Fastboot Mode by pressing the vol -and the power button at the same time and keep both buttons pressed. It is possible that you have a corrupted OS.

If none of that works consider this a hardware issue. First replace the battery. Then check the charging board and the ribbon cable going from the charging board to the main board. Make sure that it is properly seated and shows no rips, tears etc. If a new charging board and a new battery does not resolve this, the issue will be on your circuit board. For that you do need to have the schematic as well as the boardview and the knowledge to perform boardlevel troubleshooting. Most common would be failed flash memory.

Since iFixit does not yet have a specific guide for this, use something like this video