Possibly you disconnected before the import was complete. Or, after completion, you didn't disconnect. Or there was an unsupported video format that prevented the import from completing.
The usual solution is to re-import the clips, as you have successfully done. However, the new import will not contain the edits that you made to the incomplete import. Offhand I can't think of how you can retrieve your edits, other than the procedure described below.
Can you play the earlier import in your iMovie project with the edits intact? If so, you can make a screen recording of the movie as it is playing on the iMovie project preview screen, and then import the screen recording into a newly created project. You edits will be preserved. You can continue to edit it to a limited extent.
You can make a screen recording by pressing Command-Shift-5, that will reveal a control box at the bottom of the screen. You then choose the option to record a selected portion. You select the portion by clicking on the rectangle with dotted lines around it, and then drag selecting with your cursor over the preview screen.
-- Rich
