What happens if your US visa expires without travelling? Indeed, I have already travelled to the United States before. But with another visa (this unused B1/B2), some family restrictions, my wife's death, prevented me from travelling during the two years of validity of my visa. Would it be a problem to get another visa now?
Nothing happens. If anything, it helps build the perception that you don't have immigrant intent.
An unused non immigrant visa does not cause any future immigration issues. If asked for a reason in the next interview, it is perfectly alright to mention the reasons you have listed here.
I'm sorry for your loss.