is the shower fed from the main or the cold water storage tank? it could be a pressure problem, modern electric showers have a safety cut out should the pressure drop. could also be a faulty pressure sensor.
good - electric showers should always be mains feed anyway to avoid pressure drop and reduce the risk of scalding.
there is either a blockage in the pipe causing a drop in pressure look inside the shower, you may have to disconnect the feed pipe to find a gauze filter. this could be gunked up limiting the flow.
ISOLATE THE ELECTRICAL SUPPLY FIRST AT THE CONSUMER UNIT - not sufficient just to isolate at the pull cord even though it should be a double pole switch if the correct one has been fitted.
the only reason i can think of that it would just suddenly stop is if it senses a pressure drop. so it's either a blockage or a faulty sensor.
though it could also happen if someone turns a tap on elsewhere in the house whilst the shower is running - if the whole house is mains fed then another tap running will starve the shower causing a drop and the pressure sensor to kick in. eliminate this first by running the shower, wait til it is running hot then turn on another tap, preferably kitchen cold, and see what happens.