is the video directly in the same folder as the webpage that you have created with this html code? i.e www.PBSWebDesign.com/html5 there i have a seperate folder called html5 on my host. in that folder I have my index.html page and my movie.ogv file. here is how my code is on the page.
HTML 5 Video Player
<h1 align="center">This is a test of the Html 5 video player</h1>
<video src="movie.ogv" controls="controls" autoplay="autoplay">
your browser does not support the video tag
So that is the code that I am using for my video that you can go and see.
Stupid question to ask but did you maybe upload the video as .ogv and left the extension as .ogg? Anyways lemme know if any of this helps. Just make sure the video and html document are in the same folder.
edit: I know I didnt declare a doctype. I just threw this together really quick and created it all to see if it helps the OP. Remember to always declare a doctype