Bug? Some notes have no stems and can't select a single bar.

The score has become corrupted at bar 230.

I've tried to sort it out but then ran into problems with hairpins!

Basically you need to delete the bar with corruption then insert another and copy/paste the other bars after you have retyped the corrupt bar.

I was able to select all bars from the beginning of the bad bars, and paste in a new score, where most of them appear nice.

Then I can erase the bad bars and paste the rejuvenated ones..... continuing so you should recover must bars (making copy each time you have progressed).

As suggested - copy what you've got to a new score, keep backups. See if the attached is close to what you want.

...lots happening in your score:
1. As mentioned above, bar 230 is corrupted. The bar line is actually comprised of two lines, close together. Zoom in and double click to see -- also, the measure numbering jumps. Count the measures between the displayed numbers 229 and 233. There's some kind of hidden measure?
2. When things get crazy after measure 230, the pitches of the notes also change. For example, at rehearsal mark 4 (meas. #58), click on the F minim and listen to its pitch. Then go to rehearsal mark 19 (meas. 236) and listen to that F's pitch. It should sound the same. Seems like a transposing problem, too - perhaps copy/paste with concert pitch toggled? (BTW: If you toggle 'Concert Pitch' on/off the notes will fix themselves. Strange...)

Anyway, here's an attachment where all the F's sound the same. Also, now the triplets have stems and beams.
I inserted 2 measures near that double barline and then deleted the bad measures. All the other triplets 'magically' fell into place. Virtually all of the dynamics, rehearsal marks, etc. were salvaged.

Regards to all.
Sorry underquark, I'm a lot slower :-)