I frequently use Spotlight to search or start apps on my Mac instead of a third party tool such as Alfred. Since for a couple of weeks Spotlight cannot even find any of the built-in apps including TextEdit or even Safari! I am not sure but I think that might be related to the external drivers I use. Anyway, the issue can be simply resolved by adding and then removing your hard-drive from Spotlight Privacy settings. This will initiate a re-indexing of your whole system.