I've experienced this screen quite a bit in the course of learning extension development. In Chrome, right click, Inspect Element, and then go to Console. You'll find that the Tridion GUI logs some super basic events so that you can see how it's loading.
I'm sure there are other reasons, but as of right now I know two major reasons for why you'd get this screen: 1) an error in your JavaScript 2) an error in your configuration.
In order to tell which is which, you need to make a series of educated guesses. Tridion is just going down your config file, and loading and/or executing assets as it's being told to by that config. As you need to have added JavaScript for your GUI extension to work, go into your individual JavaScript files and start adding something like this after you declare your namespace, and in every function:
console.log("[whatever the name of the function is]");
When you refresh your gui, you'll find out which JS functions are executing and which aren't. If they aren't executing at all, then there's something wrong with your config. If they are executing, then you need to review the JS for errors.
And, just because the JavaScript is executing, that doesn't mean your config is fine. If one set of functions does get logged, but another doesn't, then look at where in your config you might be calling that particular JavaScript file, and this will tell you what extension in your config file has a dependency which seems to be failing.