Recently I purchase Lumia 640 Phone after one month use my phone is suddenly not wake up and when i go to service centre they told me that your phone is completely dead.
Yes! My phone is in warranty period but i want to know why this is happened with lumia phones when they are gone full battery drain is that serious issue or this is normal?
There is always a chance of getting bad unit. It happens with every manufacturer, and least with Nokia/MSFT phones. If you are in warranty and get your phone replaced, then no problem at all.