I was unable to confirm this elsewhere so I thought I should share this problem In the hopes of sparing someone of my misfortune.
I recently had a major problem with Windows. The first thing I tried was restoring Windows. My experience with Windows restore is that it only deletes system files so normally personal files are unaffected. This seems to not be the case with Arduino sketches. Once I recovered windows I was surprised to find all my work in Arduino over the past two months was deleted.
I am not certain that Windows restore is the culprit but I am quite confident that it is. If any one can confirm this I think it would be useful info.
I am well aware these should have been backed up but I think another simple way to avoid this would be to save sketches in a text format as well as the Arduino format.
Jarrad