The revocation of your POA would not be legal if the principal was legally incompetent (as you indicate he was) at the time of the purported revocation; only a competent individual may create, amend, or revoke a power of attorney. If the revocation was signed after the adjudication or determiantion of incompetency, it would not be valid; if the nursing home insists that the revocation is valid, you can bring a legal action seeking a "declaratory judgment" (court declaration or determination) that the revocation failed and the POA is still in effect. The primary evidence in the case would be the dated declaration of incompetency and the dated alleged revocation.