That`s a polarity changeover unit, does what it says on the lid. That should not stop your sockets from working, unless it`s duff. It`s a case of tracing wiring right from the inlet socket to the `van circuit breaker/RCD unit (you HAVE got one of those, it`s in the foto), if you have power to that then you need to look further. It could be that the cjangover unti is duff, or wiring has become disconnected inside.
DO NOT test the wiring with the mains connected

Keep it real.
Search first - ask second ;>}