Another possibility that sometimes happens is that their email address as presented by Google to App Inventor. For the students whose entire set of projects have "gone missing", have them check whether the email address that shows in the top right of the App Inventor workspace is different from the Gmail address they use to log in. If they are, then we can use both emails to try and find their projects.
I agree there are few students who are using the code.appinventor.mit.edu server, they might have forgot the code and done it. But I double checked with the student and I collected the codes, used by all students in an excel and they logged in with the same one. still their projects are missing.
Every student has his unique logins.
Sure I will check this possibility.