Screen turns on, blinks black and then turns off

So when I turn on my computer, my monitor turns on as well but after logging in my screen begins to blink black many times before it just gives up and turns off from the green button to a orange button( dell optiplex 760 and dell monitor btw) also my screen is zoomed in more, basically the resolution is lower and when I try to put it back to the normal resolution my monitor just shows adjusting, and states it cannot run that resolution with a black background in yellow letters. I understand this but why suddenly does my monitor become my capable to run my regular resolution? Why does it just keep blinking black and then just goes black? My CPU is running during is process and pressing keys won't make t turn back on. After a while my computer monitor just wont turn on anymore when I turn on my computer, I would have to wait a while for it to work again. Can anyone tell me the problem of this?