Racked my brain for quite a while and after lots of searching turned up nothing except a few people asking the same question. I was convinced I'd turned off a view option but finally figured it out, the software allows the time line to be resized across the mask/position section of the pan/crop areas. When I'd reattached the window that resizing overlapped onto the mask/position area. Resizing that area will again allow you to select the masking option.
