A person can legally omit heirs from his/her estate. However, in doing so, the testator (the person who has the will) opens him/herself up to having the will challenged. If your mother mentioned you, but did not leave you anything, your case is harder than if she failed to mention you at all. The will can be challenged either way. There are very short and serious time limits to filing a claim against a will. You have 20 days to file an objection to a will being probated. Our firm handles these matters routinely throughout the state.
Although adult children can be omitted from a will, I would suggest you sit down for a consultation with an attorney to discuss the matter in more detail. If your mother's wealth was derrived primarily from your father's death, there may be other controlling documents to consider.