if i understand correctly, you still have all your music in your old computer's library.
if this is the case, you should be able to select import all again.
if it were that simple, i'm sure you wouldn't have posted though
without knowing more, the best suggestion i have is to try the steps in this article
i know some of it seems simplistic, but sometimes that's all it is
If I stop and start sharing on my old computer, I would expect to see the library for that computer under home sharing. I don't. There is nothing. And that nothing propogates to the new computer!
The Music is still there in the old computer. I think the 'Import All' command you're referring to would run on the new computer, but only if there is something there for it to recognise.
So I'm still stuck, I'm afraid.
I guess uninstalling and re-installing iTunes might work, but I'm not keen!