Afzal,
Sorry, more information is needed to try to help you:
1. Did anything unusual happen the last time the phone turned off?
2. With the phone completely off, if you connect the charger/cable, do you see the charging graphic on the phone? If so, does it show the battery charging?
3. Have you tried a different charger/cable?
4. Does anything happen if you press and hold the Power button for 2 seconds (approximately the normal amount of time that it takes before the phone vibrates to let you know that the power on cycle has started)?
Sent from my rooted, debloated, deodexed Sinclair ZX-80 running CM 10.2 using Tapatalk 4