The first thing you should do is relax. DO NOT PANIC. Everything will be alright. Take deep breaths, stand a while, go away from the problem, take a walk, assess the situation, know the timeframe for fixing it, weigh the cost, time and benefit factors in saving your data. Prioritize.
[UPDATE:06/08/2010. I noticed that a lot of you who are reading this now was searching for the error:
Buffer I/O Error on Device
Somehow you found this silly blog post of mine. Sorry about that. Good job Google... Anyway, I hate it when the Google Matrix gets a glitch and fellow Googlers find themselves in an irrelevant post. So to save you some time, I updated this blog to tell you that your Hard Disk is dead or dying. Get what you can and put it in another hard disk. I came back from the future just to tell you that~dan
If you think that you should be the one who could fix the problem, then start with Google. Google the exact error message. (I know you did that already and ended up here) Follow the conversation of people who have the same problems. Get to know the appropriate log file. If you don't know which log file to find, ask in IRC or ask on a forum.
I panicked and here's my story:
I usually shut down my PC using a hard poweroff. That is, I issue the command
$sudo poweroff -h 0
Which basically, shuts down the PC, but leaves the monitor on with some text on the screen. Then I press the power button for 5 seconds. That's bad. Don't do it.
I haven't gotten to actually shutting it down properly for a long time but I recall finding a solution somewhere for it and actually wrote about it here:
So one day, I woke up and turned on my PC and it spewed out a host of "Buffer I/O Error on Device sdb2, logical block 19191"
I googled the error, and basically got the thought that the drive was comatose and is having fits of near death experiences.
I Panicked..And I lost all my pictures.