Not being an expert in problem solving...still I would think of the following:
1. Your power cable may be bad.
2. Your power supply box could be the problem.
3. Hard drive is bad (happened to a friend, similar to your story)
4. Start-up button may be faulty.
Did you buy it under warrenty? Is that warrenty still effective?
No reason for you to suffer the suspense of finding the problem.
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