My numbers app will not access files from iCloud. I turn it on in my settings, and when I go back it is off again. What should I do?

My numbers app will not access files from iCloud. When I try, I get a message saying "To open spreadsheets from iCloud Drive, go to Settings, select Numbers, then tap Use iCloud" When I do this, and try to open a document I get the same message. When I go back to settings it is off again, no matter how many times I turn it on. I have used this in the past to access my auto log to keep records of my gas expenses, and it suddenly stopped working. What should I do?