Why didn't my contacts back up in iCloud?

A few months ago I had to take my iphone 7 in for a battery replacement. I selected "back up now" before I took it in and even did it a few times to make sure. A few days ago the phone completely died and when I took it in they said the main board failed. When I check iCloud, none of my contacts from that phone are there. Do you have to go in and manually select contacts or something and then back it up? I don't understand why it didn't back them up. Thank you.