The principal (the person who created the power of attorney) can always override the power of attorney, even if they are incapacitated.
The agent (the person who was appointed to act on behalf of the principal) can be removed by the principal at any time.
If the principal is incapacitated and the agent is not acting in their best interests, a court can remove the agent and appoint a guardian or conservator.
If the power of attorney is not valid, it can be overridden by the principal or by a court.
If the power of attorney is only valid for a certain period of time, it will automatically expire after that time.
If the principal dies, the power of attorney is no longer valid.
If the principal becomes bankrupt, the power of attorney may be terminated by the court.
If the power of attorney is challenged by someone who believes that it is not valid, a court will decide whether or not to uphold it.
The specific laws about who can override a power of attorney vary from state to state.
It is important to consult with an attorney to understand the laws in your state.