I upgraded from 5 to 5.71 and now cannot see the pictures previously imported from an external HD, they are still on the external HD. Will I have to reimport them to see them in LR?
Do not re-import!
Most likely, you have created a new catalogue. Try menu item File->Open recent and open each of the catalogues shown there until you find the right one.
Hal, thank you so much, this worked:) Have a great day!