Black Screen with Cursor + Weird Error upon startup

I understand I probably should've  posted this on the Microsoft website but I am unable to sign in.

I own a Dell Latitude E5440. I run it without the battery as my previous one died.

My laptop shut down after I accidentally removed the adapter and upon restarting it, windows updated. After the update was finished, it restarted and after the windows logo disappeared a black screen came up. It was taking ages to do anything so I kept trying to restart it and after a few restarts it came up with this message: "The system detected an overrun of a stack-based buffer" and something about a malicious user being able to get in to the application. Even after pressing the OK button or X button the black screen with the moveable cursor still remains.

Even with the black screen, I can't do ANYTHING because I haven't signed in yet. I can't open task manager or anything.

This problem is causing me great amounts of stress and I have school work to be finished. Therefore, I need a quick and easy fix ASAP!! Thanks