It sounds like the photos have already been imported into Lightroom, and should not be imported again .
Do you have 'Do not import suspected duplicates' checked in the right column of the Import window under File Handling? That's a good thing BTW. If you uncheck that you could import the images that are already in your catalog. That's not necessarily a good idea though.
Some photographers will download images as they shoot... they'll pop the card out of their camera, download and then put the card back into the camera and keep shooting. The "Do not import suspected duplicates" keeps the photographer from having to sort out what was already imported previously.
If you search on one of the image numbers you may find that it's grayed out because as DJP mentioned it is already in your catalog.
Looks like that's what it was. But I don't remember importing them, though.
After i deleted all the pictures from my collection in Lightroom, I was able to import my CR2 files again.
While that might be a solution, it is a sub-optimal solution. When you delete photos from Lightroom and re-import, you wind up losing your edits and user-supplied metadata. The better solution is to find the photos in Lightroom, and then you don't lose your edits and user-supplied metadata.
Had you turned on the option to automatically write metadata to XMP before you deleted the photos, and then deleted the photos from Lightroom and re-imported, you would not lose your edits and user-supplied metadata, but you still lose the collection membership of your photos.
Guessing you meant that you deleted them from the folder... images as seen in your collections are Virtual Copies. It's a good idea to keep 'Dont import suspected duplicates' turned on!