Can I Freeze It While I Work On A Bar?

Editing is driving me nuts.

I have a 32 bar piece in 4/4 and something changed the first bar from a minim and four quavers (half note and four eighths) to a single note - semibreve as we'd say.

'Something'. I swear I did nothing. I added two bars at the end for the purpose of the second ending. I hadn't realised we probably should call a 32 bar piece a 34 bar piece if it has such an ending.

But I followed instructions and it all seemed to work okay. And I put the notes in those bars. Nothing complicated and they seemed to go in alright.

Then I saved it and had to shut down for a while.

When I got it up again that bar had changed.

So I got into editing it. And I won't weary you with the details. Enough to say it doesn't go smoothly at all. But I could get there, I can handle it, if only it didn't run onto the next bar and screw it up, too... I don't know where it gets the idea, I don't know what it's doing but it's taken an eighth note off the next bar now... am I going to have to rewrite the whole thing?

Can't a bar be isolated while one changes the notes in the bar?