Why isn't my project working?

The most common error seems to be calling your file "Index.html" instead of "index.html".

Did you include a file called "index.html"? This is the file where the user will start off. Without it, we don't know what page to look at first, so your project won't show up at all. Note that "index.html" must be all lower case letters!
All of your files must be in the SAME folder. If you put folders inside of other folders then things may not work properly.
Make sure your links do not include path names from your own machine. For example, to include a picture called "dog.jpg" we should see src="dog.jpg" not something like src="C:/Users/Dave/Pics/dog.jpg"
Be sure that you actually included all of the project files in your submission. Remember that you can download your submission from the submit server to verify that the files included are the ones you intended.
I'm noticing that this web server is VERY picky about case distinction! For example, if you have a link to "fish.html", but the file is actually called "Fish.html" then it won't load (even though it may work fine when you view it locally on your own machine).