If your machine has not been professional serviced recently or you have not cleaned and oiled it lately, then you might refer to your owner's manual instructions for oiling. I would suspect that perhaps the oil has hardened and is preventing the machine from operating properly--perhaps some of the selector lever/dial mechanics are not moving internally. Use liquid Tri-Flow Synthetic Oil (usually available at the hardware store) and put a drop or two every place that the manual recommends. Direct a hot hair-dryer into the internal works of the machine. (I usually let it get pretty warm to the touch - but make sure to stop periodically because your hair dryer may overheat.) Test your machine. If you can determine that the selector lever/dial mechanics are not moving, you can try putting a drop or two of oil in its metal joints and working it gently back and forth to loosen. Depending on how hardened the oil has become, you may have to repeat oil/hair-dryer process several times before things loosen up. Once it is working again, be sure to clean and oil regularly. (I recommend a good quality oil, ie Bernina oil for mechanical machines. It may seem expensive but it lasts a long time if you use only one or two drops each spot and, IMHO, is worth it in the long run. I do NOT recommend 3-in-1, WD-40, cooking oil, or the cheap sewing machine oil from fabric departments.) You have a wonderful machine and it will continue to serve you well if you treat it well. Suggest cleaning/oiling every time you change a bobbin, every 8 hours of continuous use, or every six months if it is not in regular use.

Take the machine to an authorized Bernina dealer for service - make sure to fill out the service slip with any problems you are having.