If there is no display at the monitor, this could be the nav unit, as this controls video, and will also overide a TV module, if present.
Generally, head unit failure is uncommon, the usual culprits are nav unit, DSP module, radio module.
Connections made or broken whilst driving are likely to be a loose plug or earth.
Radio modules, particularly BM54, are known for certain groups of circuit board soldered joints failing. There are pics of these around, and there are plenty of references to this issue if you search 'bmw BM54'