The first thing you want to do is asses the situation. Make sure that you aren't over reacting, or being unreasonable. Too many times we allow emotions to get involved decisions (personal, business, political etc) that should be made with a clear head. If you aren't sure, consult with someone (who isn't a cheerleader) and make sure you're being wronged.

Now if you are being wronged and have to take action then refer to any official paper work such as partnership or operating agreements. Hopefully, you have a document that dictates what can and can't be done as far as the business is concerned. Professionals from attorneys, to consultants, to accountants, to financial planners will tell you that you have to have some type of document that spells out the do's and don'ts when two or more people/entities are involved.

If your document addresses what to do in your current situation then that's what you go by. If there is no document, or the document doesn't address the issue then take these steps. First discuss the issue. If you and the other person can agree on a resolution, you document it and execute it. It may be as simple as changing the way you do a specific operation, to placing someone on the board to be a tie breaker to one buying the other out.

If you can not come to a resolution, then outside assistance is necessary. The best resource would be an arbitrator or mediator. Make sure you one who specializes in or has worked in business.