In describing a business as a partnership, it is implying that there are two 50-50 business partners. If it is partnership, then you have a right to dissolve it under the Uniform Partnership Act, which has been adopted in or influenced by the partnership law of almost every state. Even if it’s not a partnership (e.g. if it’s a corporation or limited liability company) you may be able to dissolve the business or extricate yourself in some fashion, but you will need to check your state’s laws for your exact rights.
You’ll have to go to court, assuming you and your partner can’t work through the matters on your own. If you want out of the partnership, the court will force your partner to buy out your interest, and if he or she can’t, then the business will be liquidated and the proceeds distributed between the two partners.
Of course, a significant problem is valuing any small business, let alone a partnership, where the ownership is closed (only two partners) and completely illiquid. While there are various mechanisms for valuing a business—such as based on assets or based on either gross revenue or operating profit—with small businesses, there’s a considerable amount of guesswork. There can also be significant expense, especially if the two partners disagree as to the value: in that case, various financial experts or consultants, accountants, etc. would need to be brought in to evaluate the business and help determine what it would be worth.
So, you have a right to get out of or dissolve the partnership, but if it’s contentious between you and your partner—e.g. you can’t come to an agreement about your leaving and how you’ll be paid for your share of or interest in the business—there is the potential to expend a significant amount of money on litigation, including on experts to determine the value. Therefore, it is very much in your interest to work something out with your partner without going to court.
You should consult with a business attorney. The lawyer can review all documents pertaining to the creation of the partnership (i.e. the partnership agreement) to see how they will affect the outcome, explain the steps of dissolving the partnership and your rights in detail, and also can help you negotiate with your soon-to-be-ex–partner.