Sickle cells are named so because of their characteristic shape. what problems can this shape cause

Red blood cells are smooth, flexible, and roundish or disk-shaped. They don't have any holes, and can move around and through blood vessels easy.
The cells form this sickle shape when they lose oxygen. Cells that have a sickle shape (which is the same shape as a crescent moon) are sticky instead of smooth, and they're stiff instead of flexible. This makes it harder for them to go through blood vessels. Because they're sticky and stiff, they can block the vessels and therefore blood flow. Blocking in blood vessels can cause pain and potentially dangerous problems.
The spleen will also destroy sickle shaped red blood cells because of their shape and lack of flexibility. This is bad because the spleen is a filter for blood and helps to get rid of things like infections. When the sickle shaped cells are destroyed, there's less red blood cells circulating blood. If less blood is circulating, there's less oxygen being carried as well. They also damage the spleen when being filtered.
People with sickled cells are at risk for infections and anemia.

Fun fact: Looking at genetics, the evolution of sickled cells actually confers a benefit to the person. The sickle cell is caused because of two faulty copies of a gene. But, through evolution, the same faulty copy of this gene provides the person with some resistance to malaria! It seems that a person with the heterozygote of this gene would have the most advantage depending on their environment, such as West Africa where malaria is pervasive.

Hope this helps!