No, without an actual written contract for a definable term, your position as a volunteer may be terminated at any time, for any reason, without prior notice, and even in violation of their own preferred internal policies. Only an enforceable contract gives you any rights to the position. And even if you had enforceable rights, if you were not being paid, it is difficult to see what you would sue for, since you can only sue for a provable, quantifiable loss directly caused by this action--and you were a volunteer (not being paid).