Different types of Viruses can hide your files and folders permanently.These viruses usually come in systems via USB or Internet and they make ur files/folders hidden even if they are removed through antivirus scan.
Dont Worry! Your Data is just hidden....Dont think that u lost all your data....
They cannot be unhided simply by 'Right Clicking' the file/folder and unmark the hidden option
So for that there is an Easy and Short way
Go to Your Start Menu
For Windows Xp : Go To 'RUN' And Open "cmd"
For Windows 7 : Type "cmd" in the search bar
Go the the specific Folder where the file is and type
To Unhide:
attrib -s -h -r folder name / file name
To Hide :
attrib +s +h +r folder name / file name
Suppose your Folder is in C:\
And the name of folder is "New Folder"
attrib -s -h -r "New Folder"
And press 'Enter'
For Your Information
S refers to System
H refers to Hidden
R refers to Read-Only
Cheers!