This usually occurs when the spooler service on your computer crashes (for whatever reason). Usually, the spooler service will crash if you have a print job go bad or you get an error message during a print job. The problem is usually driver-related.
You can verify this by going to Control Panel | Administrative Tools | Services - scroll down until you see Print Spooler Service. If your printers have "disappeared," chances are the Print Spooler will not have "Started" in the Status column. Many times you can restart a stopped service by right-clicking it and choosing Start from the menu. Be advised that you need to be logged on with Administrator privileges to do this.
If this continues to happen, delet and reinstall the printerwith the latest drivers.