PC wasn't turning on and turned itself off once

Oct 11, 2015
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I just got done opening up my computer to switch my hard drive and odd to sata 3 ports instead of sata 2 as I had just learned the difference. When I went to turn my computer back on I plugged in the psu to the outlet then flipped the switch on the psu, after that I hit the power button It did not turn on I hit it again then the pc fans started to spin but a second later they stopped and it turned itself off. I then opened the case made sure everything was in place then closed it. I then hit the power button nothing happened I waited hit it again then it began to boot, but the boot took much longer then normal and sat at a grey screen for about 15seconds before asking me to login. Everything seems to be working fine now but I am very worried about what went wrong. Could something be damaged? Please help
Thank you ps sorry for bad English