What could perhaps explain is a busy track appearing in the middle, which causes Pyramid to run out of memory while loading the project, and everything towards the end appears lost. There should be a marker on the screen (exclamation mark iirc) if this is the case. Of course there could be bugs too (eg around out-of-memory handling, it sounds like a large project anyhow), there’s no telling…
Take a deep breath (and a backup) now, and have a look at the project files on a computer. That’ll tell you whether the work is actually there or not. The rest depends on that.
And yeah, nobody cares about backup recommendations until something like this happens. I guess quite a few of us have probably been there on way or the other (not talking of the Pyramid specifically)