Who was Mr. Loyd ? Why was he called to Gateshead? How did his coming there change Jane's life forever?

Mr. Lloyd was the local apothecary. He is called in after Mrs. Reed thrusts the terrified Jane Eyre back in the red room, where she fears seeing the ghost of Mr. Reed. Jane has a "species of fits" and passes out unconscious.
An apothecary was similar to a pharmacist and was also used for low-level medical duties. As the narrative notes, Mrs. Reed is showing her lack of caring for Jane by calling in an apothecary: she summons a high-level physician when she or her own children are sick.
Mr. Lloyd's coming changes Jane's life forever because he asks her many questions about what caused her fit and why she seems to be so unhappy. He is a sensitive enough person to grasp she is in an abusive situation, despite the wealth and splendor of her home. He asks her if she would like to go to school, and when she says yes, he has a talk with Mrs. Reed that results in Jane being sent off to Lowood School. At the school, awful as it is, Jane is able to find love and friendship.