My visual editor has dissapeared on ad new posts and pages.

Hi I have recently joined and have uploaded the WP MU Dev plugin and it seems to be working but now when I try to create a post the visual editor is not visible and the html tab is not working and you can't even paste any info into the content area to create a new post. This has just happened and don't know how to fix it. Can you please help because I can not create any new posts or pages without this basic editor.