My electric shower runs out of water!?

I got into the electric shower (Mira Sport) this morning but the water didn't get hot. Waited a minute and then it just stopped pumping water out at all.

When I switch it off and leave it for a while the unit seems to 'fill up' with water again which means when I start it it pumps water out for a few minutes before drying up to nothing again. The water remains cold.

Any ideas what is wrong? The shower worked fine last night when the girlfriend showered!