Not alle Collections are migrated to Lightroom

I went through the migration, but had to restart some times- as PC hang a few times. Finally the migration said it was Complete. I have the photos in Collection migrated but Collections where the were are gone/missing
But;
I have a lot of Collections that was not migrated. I cannot restart migration again. Or can I??
Can I rebuild(aka reindex) Collections only?
How to rebuild Collections agian, or
