The painting is around 350 years old and suffered some wear and tear as well as a restoration that wouldn't pass muster today. It went through another restoration in 1994. I failed to study the painting at really close range (the price one pay's at a popular exhibit that attracts lot of viewers), but my impression was that its surface was in pretty good shape.
As for the most recent restoration, here is some information and here is an interview with the man who directed it.
And it seems that there is more than one way to do a restoration. Nowadays it's possible to do the deed digitally, as this site indicates.
Below are before and after images of "Girl with a Pearl Earring." I'm not sure about the source of the "before" image -- there were dozens of duplicates on Google. The "after" image was taken from the Mautishuis web page, so I assume it is a correct representation. Click on the "Before" image to enlarge; the "After" is at maximum size.
Restorations are necessary at times. The question is whether or not they represent a "necessary evil," given that the hand of the restorer is not the same hand that painted the original version. I suppose we have no choice but to take the validity of restorations on faith.