Dell Optiplex 780 SFF dont starts

Hello, i have a Dell Optiplex 780 SFF with 4 gb ram, Intel Core 2 Duo E8400 and 250 GB HDD. Yesterday I installed the Nvidia GT 710 video card and when I was testing it, it shows an error and the computer turns off. When I turn on it, it don't start, only shows a blinking line. I don't know what to do, please help.