Hi @Susamita, sorry to hear about this. 
I'm not quite sure that I understand your question completely, would it be possible for you to elaborate a bit on when this behaviour occurs?
If you want to raise the amount of time that it takes for the device to automatically turn the screen off, you can do so in Settings > Display > Sleep.
Let me know if that helps and I'm sorry if I misunderstood your question!
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