As ffrank says, the mirror knob needs to be in 'L' for it to work, you also need to set where you want it:
Ignition on.
Knob set to L.
Put car in reverse.
Set where you want the mirror to be when in reverse.
Take car out of reverse.
Switch off ignition and mirror should return to normal position.
Sometimes this is preset (ish) but not always.
