Your cat needs to be examined by a veterinarian. They will determine if she is truly constipated or unable to urinate. Urinary obstruction is fairly uncommon in female cats. An x-ray will be taken to assess the size of the colon, determine if constipation is present, and how severe the constipation is. Radiographs are also an initial screening test for bladder stones. It is possible a bladder stone is blocking the exit for the bladder. Both of these problems will need fluid therapy to help hydration. Treatment will depend on problem diagnosed.