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My Pentax ISTD S Camera Will not take a photo, it keeps telling me 'AV--', can anyone help?

Hello,

I have a Pentax ISTD S Camera but it is showing 'Av - -'  on its screen and will not take a photo.
I have looked at the instruction manual but it doesn't say what to do to turn it off.
It would be great if some one could tell me how to turn it off???
Thank you.