Do you have 12V power otherwise when the engine isn't running? It sounds to me as though the house 12V system isn't working from the house battery or the converter. That may be as simple as the battery disconnect switch being switched off, or a broken battery cable (possibly a ground cable), or a fuse somewhere.
If it's only the monitor panel that's not working, then there's apparently something wrong with the power supply for the panel itself. Usually, though, they're just fed from one of the usual house 12V circuits. It should be fairly straightforward to diagnose, assuming you have a usual sort of setup.