have you attempt both soft and hard reset method at first place? Try formatting both memory- phone and mass memory (you might want to backup all your data before performing this) I had the same problem once with my previous symbian 60 phone but not yet with my n95. It only happened every time my phone memory becoming too low because I used to store up to thousand sms/ mms, calendar entries etc before.
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Finally I can managed to open the back casing. I thought it's difficult and I might break it lol now it's confirm that my n95 is a new set as everything inside looks new and smell like new.. There's no bau alat2 elektronik yg dah hangit sbb dah pernah digunakan'

I suggest u to take it to the nearest Nokia Service Center to have it checked...