Upgrade from D6 cause unability to edit existing node
If there was changes in the fields you were using in the very nodes, that renders nodes unusable, you will need to export data from 6.x site to some more or less universal format, like csv or xml, and import it anew to 7.x 


Delete migrated data and it's content type
Create new content type with new-style fields, that maps 1:1 to old node structure. If possible, keep names
Export node data from original site (recreated from backup if needed), using Node Export, Views data export or similar tool
Secure all the files referenced by nodes
If needed, manually upload secured files to new version; see next step
Import nodes using tool like Node Export or Feeds - it may be required for files to be already uploaded, read the manual of your importing tool