Actually short is not always good
Like we need to know what version of iPhoto you have - details like what you mean by no thumbnail (why do you even think there is a photo if there is no thumbnail) - what happens when you edit - if there is no thumbnail; how do you "get info" - what path does the finder spotlight show for the photos?
Since we only know what you tell us details and facts are absolutely critical
One general guess is to backup your iPhoto libraray, depress the option and command keys , launch iPhoto and rebuild your database
Latest version, running in Mountain Lion. When viewing the pictures in the Main Window, one gets thumbnails (that can be resized with the slider). Those only show a dashed border where the picture or movie should be.
I rebuilt the database, and now spotlight cannot find them. The database is on a secon external HD. Since I do not know when it happened, I would be hard pressed to try to get them back with Time machine.