Is this download from the Apps Store or through Software Update?
If this is from the Apps Store, then you're using an Intel-based Mac, and this forum is dedicated for pre-Intel based Macs, so you might get more responses in the Intel-Based Mac forum, here:
Or the Apps Store forum, here:
If you're not using the Apps Store:
Maybe there was a Internet conncection error, I don't know. I've no personal experience with pausing and resuming downloads. I have had problems where downloads seemed to stop half-way through, so I quit the download, and restarted it fresh on my own.
I've heard some people say they have better luck using Firefox with a download manager or Chrome when downloading large files.
Sorry, I'm not more help.