Smoke coming from the microwave isn't usually a good sign. Over cooking items may also cause smoke in the microwave. If it hasn't caught on fire and damaged the motor you should be fine. If the smoke is not from an item you placed in the microwave, it might be time to replace it with a new one.
Your Microwave is designed to release microwave energy which turns into heat energy upon contact with food, hence warming/cooking it. Smoke means that one or more of its components may be damaged. Consult a local electrician or the manufacturer/vendor to determine if the microwave can be repaired.
Smoke coming from your microwave isn't a good thing. More than likely the transformer has burned up. You would have to have it repaired by qualified technician, however, it is probably cheaper just to buy a new one.
If it isn't coming from something being over cooked then it probably needs to be replaced. Something is overheating and will short out or catch on fire.
