It will very likely do nothing. Most hard drives are covered in a thick steel or aluminum case, that most household magnets will not be able to penetrate.
A hard drive has platters that have a magnetic coating where the information is stored. These are not magnets however. They are coated metal or glass. also, the platter spins at amazingly high speeds...
A hard drive is the main component that the computer relies on. On the hard drive, the operating system is stored, also includes your pictures, videos, music, documents, etc. No operating system on...
It is not hard to recover your hard drive if you have a back up to your files. If you are willing to spend money you can fix your hard drive by visiting Geek Squad.