It depends on the degree of genetic defect.
Normal hemoglobin is made up of four "subunit" proteins -- two alpha units and two beta units. With sickle cell gene, the beta units are defective and has wrong combination of amino acids that will result in the shape change.
People with only sickle cell trait will have plenty normal hemoglobins to carry on normal life. But extreme conditions may bring about SCD. People who have inherited the trait from both parents, on the other hand, will have crises often and may have various complications like stroke, anemia, etc.
The trigger for crisis is lack of oxygen; it can be from dehydration or infection, or... That's all I can think of right now... Anyways, when enough hemoglobins undergo shape change, the RBC will sickle. And when there are enough sickle-shaped RBCs, blood vessels will get occluded causing pain and they will also hemolyse easily causing anemia.
This is is by no means comprehensive but I hope it helps!
