I don't understand how your ex-husband could buy you out and NOT take you off the deed. Did the ex-husband pay you? Did he have a lawyer when this was done?
Regardless, if your ex-husband did not take you off the deed, then the divorce would cause you and your ex-husband to own the property as tenants in common. So technically, if you are on the deed, you still own 1/2 of the property. The other half is owned by your ex-husband's beneficiaries if he had a will or by your ex-husband's heirs (his wife and any biological or adopted children of your ex-husband). What to do is a different matter. Morally, if you were paid for your share, then your husband's heirs should own it. Legally, however, is different. It may depend on if a mistake was made and whether you knew about it or whether you caused it. It also may depend on when this was done or should have been done.
When did all of these transactions of your ex-husband supposedly buying you out occur? Why wasn't this problem discovered earlier? Did you know of the problem at all? Since I don't know what was or wasn't done, its unclear as to what should occur now. If you were paid and your husband was just negligent, that is one thing. If he relied on a lawyer and the lawyer was incompetent, its another. I suggest that you go and see a probate/real estate attorney in the county where the land is located. Pay the lawyer to review the deed to the land, any court orders mandating the transfer from your divorce and the will, if any of your ex-husband or a list of his heirs if there is no will.