Contacts randomly disappear

How is she managing her contacts? They are either managed by iCloud or another email account, such as Gmail, Yahoo or an employer Exchange account. If something happens to one of those accounts, it can affect contacts. She first needs to verify where she is managing contacts, and if she is managing on more than one platform, when she opens Contacts, check in the upper left corner to see if she is looking at all contacts.

I would suggest that she log into iCloud on a computer and check to see if the contacts appear there. If this is something that has happened in the past, trying to use the link I provide here may not allow the recovery of older deletions if you try to use it. Read and decide. The attempted recovery may just leave you where you currently are and not be successful. There is a section specifically for Contacts. If you accidentally deleted your calendars, bookmarks, or contacts from iCloud - Apple Support