How to switch on lumia 720

mohantysujata30 you have not told us if your phone charges or if you checked your charger. Make sure that it is working by measuring it with a multimeter. If it is, then start with a new battery. It is entirely possible that your battery is no longer providing the power needed for your phone. Once that is done and your phone still does not charge, take a look at the power switch and corresponding flex cable. For that you will have to disassemble your phone. Use these guides for that.