My computer screen keeps going blank and then going back to the box where I put my password in to start my comp. Why is this happening?

My computer (Macbook) was working fine this am. I left it for an hour and then came back and the screen was black (always does that as the screensaver hardly ever works). I moved the mouse to wake it up and everything was normal. I went to adjust the volume and the screen went blank and the main page with the box where I put my password in when I first start my comp appeared. I put my password in and everything was normal again. I opened a window in Safari and before it loaded completely the same thing happened, the screen went blank and went back to the password box. Does anyone know why this is happening? I don't have an external hard drive or any blank flash drives so I can't back anything up right now and am afraid I'm going to lose everything. Thank you