Question: My message is regarding a time-sensitive issue. It's not a complicated question though, as it just requires a yes or no answer.

He read my message on the chat program we use, but he did not make a reply. I sent a follow-up message a few days later, but I'm worried about what to do if he doesn't respond to this follow-up message either. We all work from home, so it's impossible for me to talk with him in person. To set up a meeting with him, I have to message him, which wouldn't work because he's not replying in the first place. Calling him out of the blue may not be appropriate. I'm going to assume his answer is a "no" if he continues not replying to my message, but I don't know if this would be a safe assumption to make.

What would be an appropriate action to take in this situation?

Answer: 
Trust me – **managers are extremely busy.** Just ask again. Follow-up as seems appropriate. Especially, don't "over-think" the fact that you maybe didn't get an answer as quickly as you expected. ("You really have **no** idea, until you've actually held that job title!")
> 
> "What should I do if my manager ignores my messages about a time
> sensitive matter?"
> 
> 
> 

Move on. Don't make assumptions. The responsibility is theirs, not yours.