Sorry it has taken so long to get a reply, but two things, a major break like this usually means someone has "done something". We have seen this range from shifting a drive letter on the server to replacing the server altogether. The other alternative is that the moodledata folder, where images are stored, has been moved or deleted or the database that contains the alias' linking the Moodle pages to the images has been changed or altered or some other event.
If it is all the images, including theme images, then you have a major problem, and best start hunting it down. If it is just uploaded images, then start at the moodledata folder. Sorry I can't be more specific, but not a lot to go on here.