When the server won't start.

• Read what it says. The page goes through the log and names what it can: a plugin that doesn't fit, too little memory, a world from a newer version. Where there's a next step, there's a button for it.
• Watch the Console tab while it starts. It shows everything the server says.
• Modded servers can take several minutes, and a first boot takes longer because the world is being generated, so give it time before calling it stuck.
• If a plugin or a mod is to blame, take it out on the Plugins or Mods tab. If it keeps failing, change the version in Settings.

Still stuck? Write to support@octocarbon.com with the server's name and what its page says.