If the will completely omits mentioning natural children at all, there is an argument that your father may have been either misled or was not of sufficient testamentary capacity to make a valid will.
Please appreciate that your father would not be REQUIRED to leave children anything in a will in most states (and I believe TN follows that rule) and spouses do have rights to estates despite wills (again in most states) but the bottom line is that although there are no guarantees here, it behooves you to consult with a local attorney (in the county your father resided in at the time of his death) to determine if you have basis to challenge the will.
Do not delay as there will be statutes of limitations to file challenges.
Yes, you can contest the will, but not on the basis that your father did not have the right to disinherit his children. In Tennessee, a parent can choose not to leave any of their estate to their children. However, if you have evidence that your father was not competent at the time the will was made, or that he was under undue influence, then you can contest the will. Either of those conditions are sufficient to void the will. If the will is voided, then the prior will springs into effect, if there is one, or the laws of intestacy come into play. Depending on how long your father was married to your step mother will have a bearing on the per centage she would be entitled to under the laws of intestacy.
The response which I have provided is not legal advice and does not create an attorney/ client relationship. Rather, it is only in the form of legal education and is intended to only provide general information about the matter within the question. There may be important facts that if known could change the answer. You should always seek competent legal counsel regarding any legal question you might have. Circular 230 Disclaimer - Any information in this response may not be used to eliminate or reduce penalties by the IRS or any other governmental agency.