Your files are not in Lightroom. They are in folders on your hard drive. Lightroom simply points to those photos in their various locations. If you want to start over then you can go to File->New Catalog and create a new empty catalog and start the import process all over again. Just be aware that any adjustments or other work you have done on those images will be lost. You will truly be starting over again.
If you want to start over you can create a fresh catalog. On the top menu click:
File >> New Catalog
Choose a new name and save
When the empty catalog opens import from your folders using:
Import >> Add