unreadable images and dark circle on screen
you are suddenly getting only a blurry round spot from your overhead
projector and no readable image, your fresnel lens is probably
installed upside down. While this won't happen by itself, it's
a common problem for those who have tried to clean the dust out
of their overhead recently. The typical overhead projector has
a top piece of glass (stage glass) and just below that is a plastic
lens with lots of circular lines on it (the fresnel lens). Most
fresnel lenses are marked on one side indicating which side goes
toward the lamp. If you know how to remove it, just take it out
and reinstall it the correct way. By the way, never use glass
cleaner on the fresnel lens, it can destroy the plastic lens.
Clean the fresnel lens with a soft cloth dampened with water or
cleaner approved by the manufacturer. But no matter what you do,
be very careful because this lens is a very expensive part
of your projector.