Can you select and drag the notes back where they belong (perhaps using the Edit Filter to make sure chords are not included)?

It may be best to try this on a copy of your file for safety's sake.

How did you create or add your pickup measure? I would have added an initial measure  and then created a time signature the length of the pickup but displayed as the subsequent measure; but without seeing your pre-pickup music I could not be certain.

I've never used that feature. I can try.

I don't know what to do

Hey, I think it worked! thanks