If you are single, only your Conservator would have that authority.
Families Without an Estate Plan FAQ's
Again, if you haven’t planned, nobody can act for you until the court appoints a Guardian and/or Conservator for you. If bills, such as your son’s tuition, need to be paid in the interim, a friend or family member would have to use their savings or borrow to pay the bill.
A court would have to appoint a Conservator. Nobody but the Conservator would be able to act for you.
Your family or friends must go to your local court and have someone appointed your Conservator. Again, this judge probably does not know you and may not appoint the same person you would choose. In the appointment process, people must testify in open court that you do not have the ability to care for yourself. […]
Unless you have planned properly, you probably will be kept on life support. In most states, you will be kept on life support unless there is clear evidence you expressed wishes to the contrary; usually this requires something in writing.
Depending on the state, if your family members agree, they can make that decision. However, if family members disagree, you could be back with the local judge getting a Guardian appointed.
A local judge would have to appoint a Guardian who would make that decision. Of course, the judge may not choose the same person you would have chosen.