Unexpected system shutdown

hii All

I was woring on my acer laptop with A few windows open and loud music. Suddenly everything just become still and even i could not move the cursor. I tried ctrl+Alt+del but nothing happened. Situation continuid till 4 to 5 min and then finally sytem restarted by itself.

Why this happened??

my system
dual core processor
3 GB Ram
32 bit operating sytem
320 GB HDD

i have purchased 1.5 yr back..

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