Not sure what device you have (or what commands you have the buttons mapped to), but the 3D Connexion devices that have buttons for specific views can do this. My guess would be you hit the Right view button on the controller. A long time ago I also saw a VBA macro that would put a drawing into this type of view without the need for a 3D controller.
I believe the Front button on the controller sets it back to the way it should be. I have those buttons remapped to something else in the drawing environment on my SpaceMouse Pro, so I can't test it for you at the moment to confirm that.
As a side note, if you are using a 3D Connexion device that has the view buttons on it, I recommend that you consider remapping the buttons to other commands in the drawing environment. The view commands are basically worthless in a drawing, so they're just wasted buttons on the controller when you're doing that type of work.
As an example, I currently use this setup:
Front = Left alone in case I accidentally do what you did, so I still have the button available to fix it.
Right = Retrieve Dimensions
Top = Base View
Rotate = Projected View
Rotation Lock = Enter (gives me a left-handed Enter key, so I can repeat commands without taking my hand off the mouse or 3D controller)
