What is the best homemade way to fix a disk and what are the reasons for the given solution

Well it depends.....

What kind of disk youre talking about?, here are the 3 possible answers:

CD or DVD Media:
If it is dirty you can wash it in luke warm water and soap then dry it throughly.

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If it is scratched you could repair it with some hardware like for example "Disk Doctor" wich repairs the surface of the media, or buy some media with protective lairs you can peel off like the Kodak brands.

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If it is cracked you should throw it in the garbage ;)

NOTE*** when washing media, be careful not to scratch the top of the media "foil side"because this is where the data is written!

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HDD / Software Errors:
If some of your programs arent running anymore or arent running properly there is a big chance you lost some data. Run Scandisk, Though you will have to reinstall these programs because most data lost is unrecoverable.

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If you cant boot into windows anymore because of partitions or kernel errors you could restore your bootable partition by booting your windows xx disk and using the windows recovery console and using the "fixmbr" command at the prompt.

HDD / Hardware Errors:

If the drive makes funny noises or don't rotate anymore you can take it to a data recovery shop then throw it out.