The very first thing you should do is use the "restart" button in your control panel. Many times, this is all thats necessary to do. If you have many addons or complex gamemodes, it may not start up instantly. Make sure to give it the time it needs to start up.
If restarting the server does not help, you should try to revert any changes you may have made recently. Installed a gamemode? Try removing it or moving it. Installed a plugin? Try removing it or moving it. Once you have done this, restart the server.
If your server is still down at this point, and searching for the error doesent help, open a support ticket!