Optiplex 745 keeps turning on and off

I was given a Dell Optiplex 745 2 days ago and it worked fine up until now. When I hook up the power cable to the power supply, the computer turns on for a second and then shuts off and keeps cycling over and over.

This is the only computer that I have. Help would be appreciated and specs are in the flair.

I'm currently posting from my phone. Could a mod flair this as tech support?