I am unable to access external Hard Drive

I have an I-Omega external hard drive which I frequently used on my desktop and laptop. It started giving me trouble with a warning of running a slow performance as my USB ports are not up to the mark.
The worst thing is, before my system can detect all other external devices and suddenly, they stopped detecting my external hard drive. My external hard drive is also not showing up in “My Computer” and gives me an error – Disk is not formatted and if you want to format now?
I did not dare to click yes because I have many important data and presentations which I cannot afford to lose. So, when I clicked “NO”, I got a message stating cannot read the drive.
I hope you could help me with this problem.
