Why did my photos disappear after delay in iCloud payment?

Of course you will not get to keep iCloud storage space while not paying for it. Photos should be downloaded to your device if not paying for the iCloud service. They will not magically appear after paying as that would indicate they are still being stored somewhere in iCloud and they are not.

You can contact Apple directly here:

Contact Apple Support - Apple Support

There is no guarantee you will be able to get those photos back. I have not seen where photos are deleted from iCloud for lack of payment, but certainly no new pictures would be stored there and there would not be any iCloud backups without having the space to store them.