Make sure you click the confirmation button after uploading your document.
Check your email! If the assignment uploaded successfully, you will receive a confirmation email from turnitin.com.
You will only be able to submit your assignment once, so actually look at the preview to make sure it's the right document.
If your computer is having issues, turnitin.com is down, or you can't login-in - email a copy of the document to BOTH of your teachers. We will still expect you to submit the document to turnitin.com, but this insures you get credit for your assignment.