My broader point is that if they are really upset with you, there are things they can do to dilute or remove your share of the company. To avoid all that potential legal nastiness, I would do my best to find a way to come to a signed agreement.

Obviously I don't know much about your situation, but I would immediately return the laptop, and offer to give back half (or some %) of the shares as a sign of goodwill.

By the way, I'm assuming you did take some sort of salary or other compensation. If not, I wouldn't be as giving in terms of shares.