Reasonably you give three genuine knocks and then consider the door closed. This of course doesn't mean if a good friend doesn't respond to three random casual calls or texts over time, but three deep efforts to get in touch with them. If they don't respond after three they either won't or can't. One anyone can miss. Two, benefit of the doubt stuff happens. If they miss three they're not there, either figuratively or literally. Unless you're in some situation where it is your obligation to take ensure they are well, there is really nothing you can do at that point. If you've got a mutual contact you can reach out to and see what's up, that's your only recourse. Other than that time to just assume they're not in your life anymore and hope they're well. Sad situation, but such is life.