Some residents told harrowing stories of losing groceries and turning to homeless shelters for help when they came to NStar’s information center opened Monday in the Boston Public Library for people affected by power outages that shut down the Back Bay area last week. Jacqueline Evans, 43, who lives with her four children on Westland Avenue in the Fenway area, said she had to go to a shelter for assistance. “When you have four kids and the lights are out, you feel helpless,’’ she said.