There are many variables to contacts. When you choose to enter a new contact you get choices, like save to phone (then your carrier may have backed up these) or you could save them to your Gmail Account (my preference), or you may other choices like Yahoo, your work Exchange email. For instance, I am on Verizon. If I have entered and saved my contacts to 'Phone', then the Verizon Cloud will have them. I can log onto My Verizon account and look at my Cloud stuff. If there was some glitches with contacts, there is a built in trash folder online that I can restore back. I prefer using Google to back both my contacts and calendar events, as it, over 5 years, has never duplicated, deleted or caused any glitches. I have helped other people with their Verizon Cloud contact backup (for the same disappearing act you have experienced, as well as duplications). It can be glitchy.
You can also checked you settings for your Google Account syncing (if you selected putting your contacts into Gmail to back up). Go to Settings, Accounts, Google and see if there is a syncing error. You can also select the Google Account and see the complete list of things that since, and select the Contacts and tap to sync again.
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