Same thing suddenly happened to me when I opened Maya today.

The default scene is fine but when I open any other scene I get the the error message about not finding top or side cameras and a substitue being used. If I manually switch them back as Mike suggests, when I switch from single view to four view I get the message again and they all revert to a single view. At one point it seemed as if the scene had flipped and was lying on the grid the wrong way up.

Can anyone help as I can't work on any scene I have. If I try and import a model to a new scene it's lying down horizontally on the grid and the same error message shows up. I haven't changed or altered anything.

EDIT: Thanks to Icarus for the windows/settings and preferences/ preferences/ restore defaults settings tip. Had to do it twice before everything was back to normal but seems fine now. Nice one!