Why does my computer just randomly turn off?

Ok it started like 3 months agoo my 2 frends were staying over and we where on omegle for a laugh talking to random pepo and my computer kept turning of but it was because my friend had it on the bed and it had to be on like a desk but was omegle giving my computer viruses ? But anyway juring the night my friend saw this thing pop up and it sed wwe have found like a lot of viruses it always comes up nd I just ignore it but my friend clicked scan and now when ever I turn it on it will stay on for about 3 mins then turn of ? I havnt told my mum or dad they wud kill me please help me fix it I really need my lap top right now oh and yea I'm only 13 sooo help me please !!!!!!