Computer Locked Problem

hi everyone,

there Is user in my network who when manually lock the computer or it just comes back from the screensaver locked, the "press ctrl+alt+del" box appears as it should. however, after random minutes the box disappears abnormally, but it never comes back no matter how many times tried to press ctrl+alt+del, so she forced to restart the computer and losed her unsaved data, where I checked the power scheme was it in always on status. what can I do to solve this problem?