There are different reasons why this could occur and you may need a tech support call to really sort it out and see if anything is still recoverable. Don't update the app or the OS on the mobile device the features were collected on until you make that determination. Sync'ing data when not on a reliable WiFi is a common way to lose data. Sync processes must be highly controlled, since the potential for data loss does exist.
One thing we have seen in the past is when you have multiple edit layers available to field staff, they will sometimes be unknowingly editing into the wrong layer. Then the features later appear to have disappeared, but were later found in a different edit layer and then transferred back to the correct layer. That may not be your case, but it is worth thinking about.
Good luck.
