Estate planning lawyers often recommend that clients establish certain key documents. Among those are a Durable Power of Attorney ("POA"). This instrument allows a person to designate an agent to act for them in the event of incapacity.
If a person becomes incapacitated and has no POA, then there's a problem. Decisions and action may be needed but no one, including Grandma, may be able to address them. If the person lacks capacity to sign a POA, then the only option may be to establish a guardianship for the person. A guardianship can be of the person and of the property, or both.
So by example, take a situation where Grandma suffers a stroke. If Grandma has established a POA appointing someone to make decisions and to act for her, then the appointed agent should be able to do what's necessary. However, if Grandma has no POA and is incapacitated to the point where she lacks capacity to sign a POA, then a guardianship may be needed. Otherwise, Grandma cannot manage her own affairs and no one has the power and authority to act for her.