Sometimes users mention that they cannot use or see the virtual keyboard. Usually, they cannot see the virtual keyboard, because it's at the bottom of their screen, and either their browser window is scaled in such a way that the bottom is not visible, or there's something that overlaps the bottom of the window.
The latter case usually occurs with Apple users, because they have a menu bar on the bottom of their screen. Here are some screenshots that can help explain what to do.
If all else fails, you can always reload their browser window to try to get the virtual keyboard back on their screen.
Apple screen with an overlapping menu:
Apple screen after maximizing the browser window: