I'm missing some files on my computer containing saved images/movies/notes for school/lectures.

What are possible reasons this deleted from my computer?

I type the folder name in my Run bar and nothing appears like it should..

check your entire folder path, not just the name. you may have moved it or renamed it. open up word or whatever program used that stuff and try to access the recent files. you can also check your recycle bin

possible reasons you deleted it:
1) you misread the folder name and deleted the wrong one
2) you got drunk one day and thought it'd be fun to delete
3) you tried to hit the page up/down key on your keyboard in explorer and fat-fingered the del button instead
4) you were using the numkeypad and accidentally turned off num lock so you pressed del instead of .
5) your roomie/parent/girlfriend did one of the above
6) the program you wrote to delete a folder was accidentally doing recursion and deleted everything

my cat once filled my hard drive with screenshots....

I found the folder.

I used the program: Search Everything.