Hard Drive won't show up. Tried everything, nothing works!?

Sounds like the HDD died if it just suddenly stopped responding like that. Just a DoA - they happen now and then. Bad solder joint that heated up, slight short circuit on a IC, etc.

When you plug it in is it spinning up at all, are any lights on the case on, etc? And its gonna sound stupid - but are you 100% positive all cables are securely connected. One of my external drives has some slightly loosish connectors... one bad nudge and it disconnects.

It could even be a simple matter of the actual SATA connector in the case wasn't on all the way. Unfortunately opening the cases usually means voiding warranties... unless you are skilled at removing the stickers they usually place on it.

As far as removing data - if it is as dead as you say you probably dont have any data removal options without voiding the warranty (which will probably make returning a pain depending on the retail outlet). Options would include taking the drive out and plugging it in using another external interface, or using a giant powerful rare-earth or electro-magnet to erase it (very difficult with the shielding on new drives).

Maybe something like dban may work, but its far safer to use that on a machine without any other storage attached (easy to erase more than the intended drive). And given the circumstances (not showing up anywhere) I doubt that will even work.