It doesn't on

@richardadd43730 3 words is all you have to explain about the issues with your computer? You have to give us a bit more if you seriously expect to get your computer fixed. Please remember that we can't see or hear anything that your computer does. You are our eyes and ears and without your help, we are not going to be successful.

You are not telling us if your computer gives you any POST beeps or any diagnostic lights.

Anyhow, here is some generic stuff I'd try. Check the power supply. MAke sure that you are geting the power to it. Next remove your RAM and all peripherals and see if it turns on if you do that. If it does you know that one of those is preventing your computer from turning on.

Here is the Dell Dimension 5150 E510 Service Manual Start around page 62 with the troubleshooting.