You mean you were unable to use Command+Option+Escape commands. It must have been one heck of a severe freeze/crash if you couldn't. Generally you can even relaunch the finder or force quit particular programs. Due to the way OSX works, it isolates all ram for every program & also the finder. If any item crashes, you can simply relaunch it without affecting any other programs or the system. That's one of the things that makes OSX so superior over OS 9 & earlier or windows OS.

Anyway, in answer to your question, no there's not much you can do. What it does is rebuild the incomplete file from zero b/c it doesn't recognise the amount that had already been downlded. Unfortuntately there's not much you can do in this scenario. At best, search the topic 1st, find the sources, then redownld from the one you originally chose. Else find the sources & force resume from the library. But as you found, chances are you must start from 0%.