Maybe the server doesn't recognize the document, maybe it doesn't exist, or maybe you're not authorized to view it. Check the URL to make sure it is typed correctly. Pay special attention to uppercase and lowercase letters, colons, and slashes. A page exists, but you are not allowed to see it. If you are sure you're allowed in, try again, and this time look at the keyboard when you type.
Passwords are often case-sensitive.
Otherwise, see if there is a registration process, or information about other types of restrictions. If you are off campus and want to use resources restricted to the campus network, please read about access via proxy server. It may be a simple case of a mistyped URL, but it may also mean that the document doesn't exist anymore, or has changed its location. Retype the URL, carefully. Many servers are case-sensitive and care about upper or lower case letters and punctuation.