Ridge preservation [maintaining the jaw bone after a tooth is extracted] is often essential to having success with implants. After a tooth is lost, the remaining bone can resorb by up to 50% within the first year. Bone grafting procedures are necessary to keep this from happening.
If your tooth was extracted without a bone graft it does not necessarily mean an implant cannot be placed. The bone just may need to be modified/augmented prior to the procedure. This is determined using a 3D image/CBCT.