Hi Rose Mary,
I'm assuming you are talking about the original Bernina 830 (their newest machine is also an 830!)
If you've not had your machine professionally serviced in awhile, it could be as simple as a good, thorough cleaning and adjusting. Your machine should be professionally serviced about every 1-2 years....not a bargain service that blows out the dust and puts oil on the hook, but a service that checks all the gears for wear, adjustments, hook timing, feed timing, grease for the gears (if required), removes lint build-up deep in the machine - very thorough.
There are various gears in the 830 that control these functions. It sounds as if one of the gears may be either broken or jammed. Unfortunately, unless you are trained in machine repair, this isn't something you should attempt. When gears are replaced or adjusted, generally the timing of the machine must be reset as well.
We would recommend that you take your Bernina to your nearest authorized Bernina dealer (they'll be trained on your specific machine and have easy access to any parts that may be needed).
If you do not know a dealer in your area, you can search on the Bernina website
The original Bernina 830 was one of the greatest models Bernina has made and is well worth putting the money into keeping it running. Great machine!
Hope this helps narrow down what may be happening.
Kim & Linnette