how to fix my desktop?

i have an emachines 220 desktop, which has a problem i cant fully explain... ive had to take the main plug out because whenever i put it in the computer makes these weird sounds and the light on the power button flashes on and off and makes a noise as if it is trying to start up but cant and it just does this constantly and doesnt start up.. the only way i can turn it off is by removing the plug..anyone have any ideas? any suggestions would be great.. thanks