Images are getting lost everytime I upload them

Everytime I upload a few images and I leave to see the front-end of the website it is ok, but then when I come back to add more images the previous ones are lost. I do not see them. Even if I go to the media library they are gone. So when I click on a Gallery, the previews are there but a message comes out once I click it “the image could not be loaded”. Please help.