Power of attorney for finance (to manage someone's finances) terminate when the perso dies. Power of Attorney for health care (to manage someone's health care) allows you to dispose of the person's remains after their death. The answer to your question depends on what type of Power of Attorney you had.
You should immediately make a police report regarding any facts you have regarding the abuse of your grandfather. Physical and financial elder abuse is a crime and also subject to civil action in California.
Consult with an attorney regarding your grandfather's estate. Bring any documents, including the "Power of ATtorney" given to you. The attorney can guide you as to your rights and duties.
If the institution holding your grandfather's remains is willing to release him to you for burial, you can bury your grandfather.
Nothing contained in the information on this web site is to be considered as the rendering of legal advice for specific cases and readers are responsible for obtaining such advice from their own legal counsel. This web site is intended for educational purposes only. Michael R. Weinstein, is licensed to practice only before the courts of the State of California, and is admitted to practice before the United States District Court for the Central District and the United States Cou rt of Appeal for the Ninth Circuit. No information contained herein is to be considered applicable to legal matters in domestic or foreign jurisdictions outside of the State of California.