I just went through the probate court process for two deceased relatives. Probate Court records are available to the public, so you are free to go to the Probate Court in the jurisdiction of your Grandfather's place of death. There, you will find his Last Will and Testament. In that document you will find out if you were one of his heirs (someone to benefit from the residue of his estate). I am guessing that you were not mentioned because I know for a fact that the court MUST notify everyone who is mentioned in the will. If you were a minor, a parent could have stood in for you to receive and guard your inheritance in Trust (monies overseen by a guardian or executor of the estate until the time to be distributed, perhaps when you reached 18, 21 or 25, for example).
If you find out and KNOW you were in the will and have yet to receive your inheritance, then you go to the Porbate Court and demand a "status hearing" on where your inheritance went and when you can obtain it.
If you are not listed in the Will, then you have no right to anything, whether your grandpa told you he was giving you anything or not.
Now, IF your grandpa did not have a Will, then his estate would be divided among HIS children. If one of HIS children died before he did, then his children's CHILDREN would divide that fair share. So, if the parent who was your grandfather's child has died (as in your mother or father) before your grandfather did, then you ARE entitled to the inheritance your deceased parent would have received. FACT.