NOTE: This is being asked, not because it is a good question (in my personal opinion, it's not), but because I've noticed a lot of questions on here boil down to this question, and since I'm sick of downvoting and giving a form response in a comment explaining basic (server-agnostic) troubleshooting.
In other words, if someone linked you to this question, you are either new to systems administration and making a rookie mistake (which is okay; we've all been new to things and asked questions that seemed stupid to people who have forgotten what it's like to be new to something), or you've been doing this a while and you are simply incompetent (harsh, but sometimes the truth hurts).
The answer to this question should not mention specific webserver instructions. At most, give some information on how you'd do things under different common classes of operating system (Windows, Linux, OS X, and BSD). That's it. So, nothing specific to apache or mojolicious or whatever webserver is in use.
...Anyway, without further ado...
I set up webserver X on platform Y, and I tried accessing the homepage, but it's not loading. What gives? What's going wrong / how can I figure out what's going wrong?