The light from the object has to fall on the screen after converging so that we can see the image on the screen.
When light coming from a distant object is converged to a point or spot on a screen by using a convex lens we see the image on the screen because light actually have fallen on the screen from the object after passing through the lens.
When the object is so close to the lens that the light from the object cannot be converged by the lens to a point on the screen but starts diverging, it appears that the light has diverged or come from somewhere from the same side of the lens where the object is. Thats why when you see from the other side of the lens where the screen is, you see the image of the object on the same side of the lens where the object is,which is virtual. You can't put the screen there to obtain that image because the light has never gone there after passing through the lens. It all passed through the lens to the other side but diverged.
But you can see the virtual image when you view from the other side of the lens where the screen is because the virtual image now becomes the "virtual" object for the lens in your eyes. A real image of that "virtual" object is formed on your ratina.
You can now think for mirrors..
For plane mirrors, you can't get the virtual image on a screen by putting the screen behind the mirror.
The image that forms on your retina is the real image of the "virtual object" behind the mirror.