files not found?

[hellohowru?]

Hi i don't know what i did but I can run any of my programs because they cant find any of the files. They used to run perfectly but now they don't because i have to add all the files manually. Is there any way I can avoid adding the files manually?
For example: I still need to add <stdio.h> and everything else and its really annoying.
How can I restore all the past settings back to the way it was?