The first case from above (corrupted filesystem) is by far the most common. There are several ways of diagnosing and fixing the corrupted/missing files. If this was just a software problem, the hard disk will continue to work properly. However keep in mind that this may also be a signs of deterioration and backup daily!
Also the hard disk will need some preventive maintenance. Check it for errors every few days or at least every week. To do that you must be logged in an admin account, then right-click the “C” drive in “My Computer”, select “Properties”, then “Tools”, then click on the “Check Now” button. Select both boxes and click on “Start”. Windows will tell you that the hard disk cannot be checked now but you can schedule a check for the next restart. Click “OK” and restart the laptop. The check will take from about 30-40 min. to over 2 hours, so do it when you’re not planning to use the laptop for some time.