I changed the hostname (from something like "Ubuntu-14.04" to "mysitename.com") of my ubuntu-server. I also changed fully qualified domain name accordingly. After that I reloaded my apache2 server.
After that only the default config started to work with the default apache-page. I tried to add other configs, but they didn't work.
So I tried to reinstall apache2 (at first I'd been intended to reconfigure apache2, but there is no such command (or so I think) and I just decided to choose the simple path - reinstallation). But with the same result.
After that I tried to remove apache2 completely. I deleted every file that had "apache" in its name and installed apache2 again. Now it doesn't even show any message when I type "service apache2" in the console.
How can I reanimate apache2 and make it able to recognize configs other than the default one?
update. Ok, I just reinstalled Ubuntu. Now everything works fine.