How do I receive my inheritance from an ancestor when my mother is refusing to give it to me?

If the inheritance is worth pursuing, then it is worth discussing with an attorney. Most attorneys will be willing to work with you, in terms of a fee. This may require your taking your mother to court, however, which is never a fun thing to think about.

Without an attorney, probably not much. If the money is owed from an estate (not likely if the money was held till you were 18) - you could go to the probate court for the county where your grandfather lived and check out the file - and possibly file a petition to compel mother to pay you (assumes she is the Personal Rep). If no probate, and a trust is involved, you could, if you have enough information, file a petition with the Probate Court to supervise the Trust and cause the Trustee (presumably your mother) to come into court and account for your interest. My guess is that a non-lawyer would struggle mightily trying to prepare pleadings for either of these scenarios.

Contact the Probate Court in which the estate was opened. You may also wish to contact your local Legal Aide office.

It is, under the circumstances you defined, your money. Therefore you may have to bring an action against your mother in order to gain possession of it. If you cannot do that on your own, you will have to somehow figure out how to afford an attorney.

Do you have a copy of the will?