This means that your original media files were not in your events folder but only links to them, with the originals stored somewhere else. If you look in the events folders inside your iMovie Events folder you will probably see aliases to the clips rather than the clips themselves. If you get info on the alieases they may identify where the original was stored (or they may say the original can't be found).
You must have moved your events folder at some point which broke the links so iMovie now doesn't know where to find them. You can still see the thumbnails in the timeline because these are small low resolution files which are in your project file. Unless you can move your folders back to exactly how they were when you created the project (and on the same computer) you will have to re-import the originals and re-insert them into the timeline.
You could as an alternative try putting one of the clips manually into your events folder in place of the broken alias to it and see if it appears in the timeline when you relaunch iMovie. If this works (I'm not sure if it will) you can do it with the rest and not have to touch the project timeline.
Geoff