I have Groups of CONTACTS on iCloud and suddenly they become empty. how do i bring the contacts back to the groups ?

It can take hours to restore contacts also depends what contact data is stored on iCloud , https://support.apple.com/en-in/HT205018

https://support.apple.com/guide/icloud/restore-contacts-calendars-reminders-mme6182cfc37/icloud

See https://support.apple.com/guide/icloud/restore-contacts-calendars-reminders-mme6182cfc37/1.0/icloud/1.0

The contacts are saved up to 30 days and after thirty days they are erased from the server , and not for kept years , by the way who are group of customers ?

Once group is deleted it can't be recovered https://support.apple.com/guide/icloud/delete-a-group-or-contact-mmfba73312/1.0/icloud/1.0

Select the contacts from all contacts ( use command and A keys ) , copy them ( use command and C keys ) and paste over ( use command and V keys ) over group .

Note : the groups will never be suddenly get emptied unless they are deleted or moved again to other groups by their own .