It sounds like you burned up something in your spindle, which was never designed for milling steel or cast iron. These high-rpm self-contained spindles can handle wood, plastics, and aluminum, but shouldn't be used for anything heavier than engraving on harder materials. How fast was it spinning when it had the problem? Fortunately, those Chinese spindles are pretty inexpensive, so replacing it won't be too painful. I doubt you'll find anyone who can fix it for less than the replacement cost.