iCloud documents disappeared

If all had disappeared then you could check that iCloud Drive is still turned on in Settings, [your name], iCloud

If only some of the documents disappeared then unless you have more than one iCloud account I don't see how they can be recovered if they were deleted.

Check Settings, [your name], iCloud, Manage Storage to see if your plan still what you thought it was. ie your payment card is still valid.

Sign into iCloud.com in a browser in a computer or ipad and check there.

Recover deleted files on iCloud.com – Apple Support (UK)

Data cannot spontaneously disappear from iCloud drive

 I want to make sure you reach the right team for help with your iphone.  I recommend that you contact Apple Support directly. Depending on your region, you may be able to reach them here

Contact - Official Apple Support

or phone numbers

Contact Apple for support and service - Apple Support

Firstly which type of document? Documents in iCloud Drive, or Notes, reminders, Calendar etc?

Secondly, all documents or just some?