For nearly 48 hours last week, Hall County’s emergency 911 center relied on a generator to remain operational as a crippling ice storm brought down trees, knocked out power for tens of thousands of residents and made many residential streets impassable. While calls to 911 went uninterrupted during that time, county officials said they fear a repeat experience might not go over as well. “I didn’t sleep for ... days,” said Hall County 911 Director Gail Lane.