Hi Rosie
This is a common problem - design fault.
Unplug from the mains socket (sorry, but it doesn't seem to occur to some people ;)
Remove the screws (lots) and take off the back cover (grey metal) to expose the heater element unit.
On top of the element unit are two round electrical connectors, each with two wires attached. Look closely at the left one - there should be a pin hole in the middle of it. This is the reset switch. Use a thin shaft (straightened paper clip is fine) and push down through the pin hole - you should feel the reset switch click.
Replace the back cover, connect to the mains and dry your laundry!
If the problem repeats regularly, try washing and drying the door filter and vacuum the outlet hose and passageway beneath the filter. Don't use those scented drying powders! Finally, you may have to replace the heater element unit - not too expensive and easy to do.
Hope this helps
Ashley
Ashley, January 2007