Olympus voice recorder WS853

I have been using my Olympus WS853 voice recorder which plugs into a USB port to charge & download the MP3 recorded files & copy them onto my computer to be read by the Itunes App.

Today when I plugged the recorder into the USB port on my iMac the external disc icon for the recorder failed to appear on my desktop.

When I went to the disk utility application the recorder did not appear as an external disc, as it should have done.

What is causing this to suddenly happen.