Thyroid stopped working

I found out today I have a thyroid problem. My doctor said my level was basicly at zero and my thyroid had stopped working. He prescribed me levothyroxine 75mcg. I didnt really know what questions to ask and I feel really confused about the whole situation. What kind of thyroid problem is this hypothyroidism or hyperthyroidism, underactive or overactive? How long does it take levels to balance out? When should my body pain and fatigue start to lessen and go away? Any other advice or anything I should know would be appreciated too.