Why does my neck suddenly hurt so much?

I woke up the two days ago with what I thought was a sore back, I had slight discomfort when I leaned back or turned my head too far. Yesterday it got worse and I can tell that it isn’t my back, it’s my lower neck. It hurt when I turned my head to the left and when I bend my head back. Today it is even worse. It doesn’t hurt when I lean my head back anymore but I can hardly turn my head to the left without a sharp pain. I haven’t the slightest idea what caused this.

What is the cause of this? Why does it keep getting worse? Am I sleeping on it wrong? What should I do?