Out of sudden, it shut down itself. I tried to turn it on back, but it didn't respond. So I thought I needed to wait for a while, maybe it needs some rest. After an hour, it still didn’t recover. Remember, I did not jailbreak it. And the battery was still full. No battery problems, it wasn’t dropped at all, and no water damage.
After few hours, I looked for solutions via Google. Then I found this one trick;
Holding down power and home button for 10 sec.
Yezzzzaa!! I did it. It turned on and managed to recover.
Now still thinking what makes it temporarily shut down….:(