there are tons of possibilities. first off a CPU runs hundreds of thousands of calculations every second. if one error occurs or some numbers are given in the incorect order poof the computer crashes and may restart shut off, or freeze. this may have happened. you may have overloaded your ram and in the same way crashed your computer,
the website could have been running an avtiveX script that made your computer restart, it is possible and relatively easy but unlikely that a commercial website like a porn site would have a script like that.
there are douzens of other possibilities from random errors in the innerworkings of your computer which may or may not be related to the page you were currently on to people or programs maliciously making your computer turn off to heat issues.
I cant really tell you what happened in your case without being there. just update your antivirus and get spybot search and destroy, run both of them if it comes up empty then just forget about it.
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Everybody Knows that Christians dont beleive in Gravity
I intend to live forever, so far so good
If you were touched by an angel call the police.