Photos that are in the cloud, such as iCloud Photos with optimized storage, won't show up in iMovie.
Also, are you opening iMovie in the same user account that contains your iMovie photo system library?
For further diagnostics you could try opening iMovie while holding down the Option and Command keys and selecting to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects. Now see if iMovie will read your Photos library.
Another thing that you can try is to redownload iMovie. To do that drag your iMovie app from the applications folder into the trash, but don't empty the trash. Then sign in to the app store with your Apple I.D. and password. Open your app store account by clicking on your Apple I.D. at the bottom of the app store side bar. When your account opens you will see a list of your purchases, iMovie among them. See if you can redownload iMovie from there. If for some reason the download is not available or is unsuccessful, drag your old iMovie out of the trash and replace it back in the Applications folder.
-- Rich