Why when I turn on my computer a black screen...

Hello there! I hate to do this, but I am wondering why my computer turns on and (I don't have correct adapter but it does work) it says blah blah blah the adapter isn't a Dell one and should be using one with a little more voltage... It worked fine before regardless, but now I will hit F1 to get rid of the message then it turns to a black screen with a blinking white underscore in the top left corner.... I don't know what to do!? Any help please!