Barracuda 7200 12 device i/o error

First, before you panic, have you disconnected then reconnected it to your motherboard? Failing that, trying hooking it up to other computers, or even doing so through a USB hard disk enclosure.

It could potentially be a corrupt firmware issue for this hdd.

Try running seatools downloadable at http://www.seagate.com/www/en-us/support/downloads/seat...

Also try mhdd downloadable at http://hddguru.com/content/en/software/2005.10.02-MHDD/

The output from the above tools should give more information as to what the situation is......once you have more diagnostic info , you will know what needs to be done to get your data back.

why haven't you backed up your data?

rule #1 - NEVER expect your data to be safe on any 1 storage device