Perhaps someone from NI can contradict me on this one, but I think once the block diagram is gone, it's gone.
It's not hidden, or passworded, it really is removed as far as I know. I've been in the same position, and the only option is to re-write from memory, or re-build from an intermediate version. You do have an old version stashed away somewhere don't you?
Sorry to be the bearer of bad news (Feel free to release your frustration with a *)....
Shane.
Using LV 6.1 and 8.2.1 on W2k (SP4) and WXP (SP2)