I fell down and hit my neck, where my goitre is, it hurts really badly, what should i do?

See a doctor: The trauma in that area should be checked by a doctor and if necessary blood work and x-rays can be done. There are many important structures in that small zone.

physician exam: Hemorrhage into thyroid gland as a result of blunt trauma is very rare condition. Gland would enlargement and maybe cause hoarseness, difficulty swallowing. In graves' disease a thyroid storm where thyroid gland puts out excessive hormone causing increased palpitations, diarrhea, fatigue, etc. Can see physician for exam possibly ultrasound exam if symptoms continue or increase.