About downloading photos from iCloud I turned off iCloud photos on my iPhone. After that, some of my photos have been lost. How can I find them again and then restore them?
Sounds like the photos you lost were stored in iCloud and not on your phone, so they disappeared when you turned off iCloud Photo Library.

You can either enable it again, or find them at www.icloud.com (click on the photos app once you login).

This can also be done in the desktop photos app if you have a mac.

From there you can share these photos, maybe you could email them to yourself 1 by 1 and save them to your phone.