This is what is supposed to happen with previously imported folders. You do not want to import the photos a second time, the photos are already imported into Lightroom. All you have to do is go to the Library Module and find the photos and you can then perform any operation in Lightroom on them.
I did assume the photos are in LR as well as the folder on the desdktop. Bt when I go from library module to develop , the thumbnails are "greyed" out and I can't work on them.
The photos are NOT in Lightroom. They have been imported into LR, which means that the LR database has a record of the location of the photo, and a record of other important information about the photo, but not the photo itself.
As far as I know, thumbnails do not get greyed out in the Library Module. So please show us a screen capture of what you are seeing.
Also: you state
I can't work on them.
Exactly what happens when you try? Is there an error message?