This is a tough one. The iTunes Store doesn't provide any way for you to get back purchased items that are lost or accidentally deleted. The Store is set up to require you to buy them a second time.
Though some download services like eMusic do allow unlimited re-downloads, iTunes doesn't. I suspect this has to do with licensing and paying royalties to entertainment companies, but I'm not sure.
Before you go buying the item again, try reporting the issue to iTunes Store support and explaining the situation. It's possible that you’ll find a sympathetic employee who will give you a free download to replace the item you already bought. It's been known to happen (though probably won't if you make a habit of this kind of thing).
Don't expect it, though. If someone does that, it's more of a favor than a policy.
If that doesn't work, the hard answer is, just like a stolen CD or broken DVD, you'll need to buy it again.
