Could you tell if you see your hard disk when in BIOS? Make sure the boot order is right. Do you get some sort of error message when you try to boot(eg. “No bootable device found”)? Have you tried booting into safe mode? Do you hear some odd notices from the machine? If you get to BIOS I guess we could rule out your PSU, however it would be best if you could test all you components on a different machine. The first suspect in such cases is the PSU, so do check it on a different machine if you can (make sure the Wattage is OK with the systems – there are various calculators to be found online). If the PSU checks out next I would suggest testing your system with just the essential components – PSU, motherboard, CPU and one stick of RAM (you may have to include your GPU in case you do not have integrated graphics) That way we can start ruling out the possible causes. Also, take your hard drive and connect it externally to a different PC, afterwards, run a brand specific diagnostics tool to view the SMART status, and post a screenshot of it. There are other diagnostics tools, however, the manufacturers tool usually provides the most accurate results. Generally, providing more information would be helpful, do tell you system specs, HDD model and brand.
Keep me posted,