I feel sleepy n lethargy whole day even after having sleep of 10hrs?

Need evaluation. Many, many causes of this most of which revolves around stress. However, you could have sleep apnea, restless legs, fibromyalgia/chronic fatigue syndrome and hormone imbalances. Find a doctor that can investigate these types of issues.
Do you snore? IF you are not sleeping efficiently may have this. If you snore you are probably having apnea and a sleep study would evaluate this possibility-which could result in non-restful sleep.