Took computer apart and now it won't start

Ryan_Xray_Tech

Commendable
Jun 23, 2016
6
0
1,520
I have a pre built Dell computer from 2007 and I decided to take it apart to see if I could put it black together.. I put the components into the case and the power supply is hooked up to the motherboard, cpu, hard drive, and disk drive. However, when I plug it into the wall.. The computer makes a humming noise and off after about 5 seconds of being on.. It then turns on again and then turns off.. Nothing shows up on the monitor and a yellow low comes on the motherboard as soon as I plug the computer into the wall. HELP
 

Ryan_Xray_Tech

Commendable
Jun 23, 2016
6
0
1,520


Well I didn't come on here to be insulted.. It is an old computer that the family doesn't use cause it is just to slow.. Everyone upgraded. I'm brand new to working on computer and I'm trying to learn. I did check the cabling and BIOS won't come up because the computer turns on and off every 5 seconds or so. I'm just trying to figure out what causes the noise, the yellow light and the computer turning on and off by itself once plugged into the wall.