After upgrading my hard disk, I discovered that Spotlight (Mac's search tool) could not locate most of my files. I knew the files were still in my disk, but they would only show up in Spotlight after I opened the files.
I figured that Spotlight needed to reindex my folders, so I did a little googling and found a good solution.
Open System Preferences (Click the Apple logo > System Preferences). Click Spotlight, then Privacy. Open Finder and navigate to the /Documents folder. Drag the Documents folder into the Privacy list. Then remove it (highlight it and click the minus sign under the box). This will cause Spotlight to reindex that folder. You can even drag the whole disk into the Privacy list.
If you have an external drive, you may ask Spotlight to index it through this way too.
(Important: don't drag the Documents folder icon in the sidebar of Finder or you'll remove that icon).
For a geekier way to do this (read: via command line), read this link.