Just to give a background and information before I give the full problem.
I rent from NFOservers. We've been wanting to add mods to our server for a while, but most of them require Minecraft Forge. So, I tried installing it a few days ago. I have followed the directions: Downloading it, changing the extension from .zip to .jar, and putting it into the server for selection. Now, here's the problem. Whenever I select it, the server goes offline. Despite constantly selecting the "Start the server" option, the server will not go online. What's going on?