Project suddenly stopped working (Game not able to play in editor)

I haven't a clue what has happened here. For no apparent reason, my game no longer plays in editor. Normally by pressing the play button at the top of the editor, the main camera would shift to the POV of a small vehicle and the arrow keys would be used to control this vehicle. Now all of a sudden when I press the play button, I hear a faint sound, the grey colour of the editor changes as if the game has started, but the camera and controls still stay in editor mode. What's even more confusing is that I genuinely didn't change anything to cause this.

I am screwed if I don't figure this out because my uni coursework for Unity is due in a few weeks. Does anyone know the types of things that could cause this sort of catastrophic error? I was changing something in a script just before it happened but it really shouldn't have done anything. Besides, I changed the script back to exactly as it was. It's almost as if something was permanently overwritten.