Problem with links

I am having difficulties with my links - some do not seem to work, that is, they are not bringing up the pages to which they are linked, and I don't know why. As far as I know, the material to which I link is not part of a private subscription or licensed database. I have been able to fix some of my links - http:// appeared twice in a row in some of the code, which is why the linking wasn't working for some of them.