What can happen if a clot of blood forms?

this process is called blood clotting...

when there is an injury , the platelets in the blood comes in contact with air they decompose to form thromboplastin, thromboplastin combine with inactive prothrombin, formed by the combination of Vitamin K and Calcium ions,to form active thrombin,thrombin combines with the fibrinogen in the blood plasma to form Fibrin, this Fibrin entangle with bolood corpuscles and thus clot is formed...
Blood clots are nature's way of keeping people from bleeding to death when they are cut or have injuries. Another term for blood clotting is coagulation. When you get a cut or puncture wound, your body's blood clotting abilities spring into action even before the first drop falls. There are several main parts of the body that act in this clotting process. The major cells responsible are the cells lining the blood vessels. The inner layers of these cells are most important in blood clot formation. People also carry a protein called fibrin. The fibrin is part of a network of proteins that stick together, with the fibrin acting like the glue in the process.