Recuva is freeware and allows you to recover some files you may have accidentally deleted or lost. That said, whether you can restore them or not depends on the state of the files. Some may be damaged or even completely broken and unuseable so...best of luck bro.
To download and convert videos from Youtube, aTube Catcher is also pretty good. Used to have an option to download ALL videos from any given channel, but I'm not sure if that's still around.
In terms of the OS itself, maybe a system restore could help? Though it won't restore any deleted files as far as I'm concerned, just software and settings
![Confused :S](./images/smilies/confused.png)