Hi Kerri,
A power of attorney usually expires upon the death of the grantor of the POA. However, your mother may be considered next of kin in the state where the minerals are located. Get a death certificate for your father and ask an attorney in the subject state what to do next. If you have copies of the leases you can contact the county clerk and check the status or contact me and I will see if there are any wells holding the acreage.