Depends on where the clot is and if it's in a vein, artery, or not in blood vessel at all (hematoma).
Vein clots cause swelling and pain and may migrate to the lungs which can be lethal. Artery clots acutely impair the circulation: in the heart
, it's a heat attack; in the brain it's a stroke
; in the eye
, it causes blindness, in the arm or leg, it's painful and could lead to amputation