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I have the same one. Yeah it will have gone manual because someone pressed the plus or minus buttons to manually set a temperature.
As said earlier, press the right button to take it back into auto mode. Then consider placing it upstairs where you have the heating so it'll turn itself off when the upstairs reaches the desired temp.
Nothing broken about it, so no need to replace unless you're looking for an excuse to do so.
Oh, so it went to manual by itself? I assumed you changed the mode without realising :/