If you had an original partnership agreement, the agreement might help answer your question. If not, when it split up, was there an agreement? What was your contribution to the partnership? The real question is, were your efforts on the site part of your contribution? Were you compensated for that work? For other work? For the hosting? What do your (former?) partners believe about the site? Do you plan to sell it for a lot or relatively reasonable amount? Do they still owe you anything?
You have some choices. They are based on what you want, what you think they would reasonably give you, whether they think that you are trying to hold the site for ransom.
Although you don't necessarily need an attorney right now, it may be good to meet with a business lawyer to help you analyze your position and maybe help you negotiate an acceptable resolution. I wouldn't just turn the site off as it is more likely to make them not trust you, making it harder to get a good resolution. They might even sue you to get it back up meaning you would have to hire an attorney to defend you. That would be money wasted if you can find an amicable resolution first.
Mr. O'Rourke provides excellent counsel. If the website was an oversight, you must have done this dissolution without an attorney or with one who does not specialize in this area of law ... always a mistake. I suspect you do not have a partnership agreement or buy-sell agreement that describes what happens to the domain name and website. And unfortunately for you, it is a question of fact as to whether your partners reasonably assumed the website went along with the deal and if you were compensated, either through forgiveness of debt or payment, for the domain name and the work done on it. You aren't going to be able to figure this out on your own. You need to tell your story and show your documents to an expert business attorney who can sort out the facts and apply the law to your situation, and provide you with some idea of your options and the cost to proceed.
Mr. Huddleston is an Ohio-Certified Specialist in Estate Planning, Trust & Probate Law, with offices in Columbus and Dayton, serving client families and private business owners throughout Ohio. He may be contacted directly by phone toll-free at 888.488.7878 or by email CLH@HUDDLAW.COM. Mr. Huddleston responds to Avvo questions as a public service to help educate and provide general guidance to questioners, but his responses are not legal advice and do not create an attorney-client relationship.