I would say that some sort of corruption has occurred in the drawing file.
Whoever you have for your subscription (edit: your VAR/reseller), I would contact them and show them the issue. Sometimes they can take a file and recover the data.
On a side note, with large assemblies/drawings, I have noticed that if I switch back and forth between them too fast, then something generally goes wrong. Mainly in the drawing area. It is like Solidworks isn't quite done with its background processing, but still allows me to switch away, then starts a new set of background processing while the last set wasn't complete. Then wires get crossed and you get corrupted files.
My advice: Keep things in lightweight mode in the assembly as much as possible and if you switch between them, give it a minute before you switch back.