How serious can a Cease And Desist Letter be. What should I do when I receive it and should I stop doing business? Can I be sued?
A cease-and-desist letter is a letter demanding that the recipient refrain from a certain behavior or face legal action.
Since your situation involves a business context I would assume the letter is addressing one of the following issues: copyright, patent, or trademark infringement. Typically this type of letter threatens a civil lawsuit if the recipient continues the undesired activity.
In the US, a recipient of a cease-and-desist letter who is placed in a "reasonable apprehension" of litigation may respond through a request for something known as a declaratory judgement proceeding. This is a court decision in a civil case that tells the parties what their rights and responsibilities are, without awarding damages or ordering them to do anything.
Right now you have 2 options - stop the offending activity or consult with an attorney in your area. If you choose the latter, you should arrange for a meeting ASAP.