1. What vid card are you using?
(I use a GeForce card, and this happens all the time because it is not qualified for Maya, however, I've written a workaround-script to cope with it in a nano-sceond
2. When this happens, Deselect everything, then IN YOUR OUTLINER select something and click any tool in the toolbox.
Does the viewport appear then?
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS