Not sure what happened, but I'm working here with two viewports (2D and 3D) and out of the blue, the 3D image simply stopped working. Nothing shows even when I try REGEN or reopen the file. The 2D image works fine and *sometimes* I can rotate it to show in 3D, but if I use any of the preset isometric views, nothing shows.
Does it happen in one particular file or every file? I think I have seen similar behavior and I think it had to do with 3D clipping planes being out of whack. Try copying everything to a new blank file and see if it helps.