Computer doesn't turn on

Hi Everyone,
I have a dell desktop which had some problems and made a weird sound whenever I turned it on so I decided to open it up and take out the HDD and plug it into a desktop that I built and backup the data and reformat the drive. So then I put the HDD back in and now whenever I try to turn the computer on it doesnt even turn on. Before it wouldn't show me anything but atleast when it was turned on you could hear everything spinning as the power went to all the components. However now the MoBo lights up and thats it, I tried pressing power button multiple times but nothing. All the connections seem to be secure, the pins unbent and the parts snug into their slots so I don't understand what the problem could be. Please help!!!