Changed power supply now getting a blinking amber light

[Gretchen]

My mom gave me a Dell Dimension 4700 computer that was acting up. She said it would turn it's self off randomly and she thought it was a virus. When I brought the computer to my place, it powered on once, then I went to turn it on again and nothing. No lights, no power what so ever. So after doing some research I bought a new power supply for it. I just replaced the power supply and plugged the computer in and now there is a blinking amber light in the front and a solid green light on the motherboard but it won't turn on. I have checked all of the connections and they are properly seated. Any ideas what is causing the problem?