Hello Dona Salomone,
Welcome to Microsoft Q&A forum.
It is weird for the file to become blank when it is password protected. Before we proceed, may I know the file type and where was is stored? Also, can you please check the file size to see if it unusually small compares to how it was?
Additionally, can you please create a new test file with the same type as the affected file, write a few lines on it and create password protection, then save the files and reopen it with password to see if the lines are still there?
I'm waiting for your reply.