Assuming you are referring to iCloud, it is not 100% reliable. The safest method is an external hard drive and 1 - 2 copies of that.
I do not have an answer to the disappearance (I do not use iCloud). Do you have any second backup and have you looked at the article linked to above your name??
Here is a link to the iCloud Terms:
Legal - iCloud - Apple
and this is an excerpt:
Backup
iCloud Backup periodically creates automatic backups
for iOS devices, when the device is screen locked, connected to a power
source, and connected to the Internet via a Wi-Fi network. If a device
has not backed up to iCloud for a period of one hundred and eighty (180)
days, Apple reserves the right to delete any backups associated with
that device. Backup may include device settings, device characteristics,
photos and videos, documents, your messages, ringtones, Health app data
and other app data. For additional information, please go to What does iCloud back up? - Apple Support.
The following content is not included in your iCloud backup: content
purchased from the iTunes Store, App Store, or Apple Books, media synced
from your computer, and your photo library if you have enabled iCloud
Photo Library. Apple shall use reasonable skill and due care in
providing the Service, but, TO THE GREATEST EXTENT PERMISSIBLE BY
APPLICABLE LAW, APPLE DOES NOT GUARANTEE OR WARRANT THAT ANY CONTENT YOU
MAY STORE OR ACCESS THROUGH THE SERVICE WILL NOT BE SUBJECT TO
INADVERTENT DAMAGE, CORRUPTION, LOSS, OR REMOVAL IN ACCORDANCE WITH THE
TERMS OF THIS AGREEMENT, AND APPLE SHALL NOT BE RESPONSIBLE SHOULD SUCH
DAMAGE, CORRUPTION, LOSS, OR REMOVAL OCCUR. It is your responsibility to
maintain appropriate alternate backup of your information and data.