My Lumix DMC FZ8 will not always take a photograph?

It is as though it has gone on strike...sometimes it works fine and then for no reason at all...it just won't let me take a photograph...the battery is full, the memory card is empty, I press the button and ...nothing..it just beeps, I get a small green circle on the screen and...nothing...it just beeps but no photo is taken..
what is strange is that I can take a video...just not a photo.!!!