- Hard disk are replaced, not repaired. A brand new, 2TB SATA disk costs less than $100.
- You might still be able to read the disk and save the data using disk recovery software, but this can be hairy business, one has to be careful. Commercial data recovery services start at $500 or so.
Next time your computer warns you that you hard disk might be about to die, take the warning seriously. Get a new disk and copy your data over ASAP.
Also, do make backups of important data. As the adage goes, the question is not if? your hard disk will fail, but when?
EDIT: to answer your question about fitting a new disk, the new hard drive should be "plug and play", on the condition that it is compatible with your computer.
However, copying the data and the OS over to the new drive can be a bit of a chore. This requires of course that you fit your new hard disk alongside your old one until you're done copying. I use GParted, a hard disk management software that can copy entire disk partitions at the click of a mouse. It's free, you download it and burn it to a CD. You then put the CD into your computer's optical drive and reboot. The computer will boot from the CD, and GParted will start by itself.
If your computer is capable of booting off USB devices, you can use an USB memory stick instead of a CD.
If you're not familiar with this type of operation, I suggest you get hold of a buddy who is so (s)he can walk you through.