After completing her pre-takeoff safety check of the cabin, the flight attendant (FA) sat down in the jumpseat but reported that her lap buckle was stuck under the seat bottom and so was unable to secure her harness during the takeoff roll. After takeoff and in the initial climb, the FA stood up to release the buckle from the seat bottom and then before she was able to buckle her seatbelt, was thrown from her seat onto the galley floor injuring her side. The FA was assisted back to her seat and was unable to complete her duties. The flight returned to the departure airport where emergency medical technicians met the plane. The injured FA was transported to the hospital and diagnosed with two fractured ribs.