My photos won't load, a small cloud icon appears instead

About 2 weeks ago, I deactivated the iCloud Photo Library because some of my photos weren't therem and activated it immediately after. All my photos were uploeaded to iCloud, but since then in my photo library about 300 photos and videos have disappeared (it's blank) and a small cloud icon appears in the thumbnail. Whenever I try to see the photos, a small icon seems to be loading, but it stops. What can I do?