Where has my viewport gone?

[Merzbau]

This is an embarrassing n00b question, but please hear me out. I haven't done any level editing since the days of Half-Life 1, and even then it was pretty crude, so I'm following along with the UnrealEd tutorials and, aside from the missing wood material and dragon static mesh, everything is going smoothly. Except now that I've loaded my saved work, my top-left viewport (which I've left as the default Top view) is totally grey, with no grid lines, and shows coordinates of -2147483648, -2147483648, -2147483648 which I cannot change by click-dragging the mouse around. When I try click-dragging, instead of telling me how far I'm moving the mouse, I get (No Change) at the bottom of the screen. It's the same if I switch it to Front or Side view, only showing anything at all if I switch it to Perspective, in which case it's fine. Help!

[afx]

On the top of the toolbar, click on View -> Viewport Configuration -> 2x2 Split. This will reset your viewports.

[Merzbau]

Thanks! I have no idea what I did to my viewport, but I'm happy to know there's a reset function.