Hi,
It looks like you accidentally deleted iMovie from Launchpad. That would work to delete it. (Another way to delete iMovie is to control click on its icon in the Applications folder and then click on Move to Trash, and then empty the trash. )
Try looking in the Trash and see if you can retrieve it from there. If not there, try looking in the Applications folder and see if iMovie is still there. If not there, then try this procedure to redownload it:
To update/redownload iMovie 10, drag your iMovie 10 app from the Applications
folder into the trash, but do not empty the trash. (You can skip that step if
you had iMovie 10 on your computer before, but deleted it.) Sign in to the app
store and open your purchases folder by clicking on your name at the bottom of
the app store side bar. You will see your purchases displayed, including iMovie 10.
Redownload/update iMovie from there.
-- Rich