Restoring finder icon
Goto your Home Folder > Library > Preferences and delete the files:

com.apple.finder.plist    
com.apple.sidebarlists.plist


If you can't find the files use this method: from the Finder menu bar, select Go > Go to Folder and copy the following line into the text box that opens:

~/Library/Preferences/com.apple.finder.plist


Then, restart, or log out and in again.

Edit

Select Go > Go to Folder and copy the following line into the text box that opens:

/System/Library/CoreServices/CoreTypes.bundle/Contents/Resources/


Here is where you will find all the sidebar icons -


Now you will have to manually change the icons for the respective folders by using Get Info on the folder that is in your sidebar and by dragging and dropping the icns file onto on the Inspector panel.