I had a same problem just now, with missing media library images for my blog. The images appeared to be right there in the media library and were definitely on the actual web server (checked via FTP).
As Allen Z advised I did "check Settings â†’ Media and make sure that Uploading Files folder is set to wp-content/uploads"
Mine were set to the default blank. I altered this to an absolute path http://www.example.com/wp-content/uploads
THIS DIDNT SOLVE THE PROBLEM when I refreshed the site in browser. However, I immediately changed the path back to blank (the default setting again) and everything came back! Woop
Everyone having this problem might want to try this before getting into the more technical fixes!