[iTunes] disappearing podcasts!

[Becca]

I've downloaded a bunch of podcasts from the A History of the World in 100 Objects through iTunes. There are currently about 50 of them. I'm slowly working my way through them, but am maybe half done. All of a sudden, when I opened iTunes today, they all have exclamation points by the titles, and I can't find them on my computer anywhere. I'm afraid to sync my iPod now, for fear I'll lose the ones still on it - and I'd really like the missing ones back, because I wanted to save them. 

How do I get them back? I've tried unsubscribing and resubscribing to the podcast, hoping to get the "get" button for them, but it doesn't seem to help. I know I can download them individually from the site, but there are so many of them that would be time consuming and a pain.

any advice on how to get my missing podcasts back?

-becca

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There are nine and sixty ways of constructing tribal lays
And every single one of them is right!
--Rudyard Kipling