Hi,
Here is a link to the start of the pages in the service manual for your PC which may help you. They detail what the problem might be by giving you the meaning of the lights and beep codes if any on display when you try to turn on your PC.
http://www.manualslib.com/manual/677079/Dell-Dimension-5150.html?page=62#manual
Here is the link to the service manual.
http://www.manualslib.com/manual/677079/Dell-Dimension-5150.html#manual
Hopefully this is of some help.