You've looked in all the right places, so now we need to determine if it's a scene problem or a preferences problem or a hardware problem....

1st open a new scene in maya and create an object and see if it is visible in the orthographic views.

If you can see the ortho views in a new scene then there is something messed up in your scene file. To fix just export the objects from the bad scene and import them into a new fresh scene file.

If you cannot see the orthos in a new scene then check to see if your preferences are messed up. Rename your maya folder in your documents folder (if you are using windows. I don't where it is if you are on a mac).

Then restart a new maya scene and create an object and see if it is visible in the orthographic views.

If it is, then open your mailbox scene and see if that object is now visible in the orthographic views.

If it is then there is something that got flipped in your preferences.

To verify, close maya and rename replace the maya folder with the original one and open your scene file with the mailbox and check the orthographic views.

If they are blank again then it's definitely something in your prefs and you'll have to decide whether it's worth the time to track it down or just start with new prefs.

If, on the other hand, you cannot see the ortho views with a new scene, both before and after, renaming the maya folder then you are most likely encountering a hardware problem, usually graphics card or drivers.