Complex sentence: After losing track of Gollum, the Hobbits realized that the tunnel was blocked by her webs and she cornered them, but Frodo used the Phial of Galadriel's light to drive her off.
Split into simpler sentences: After losing track of Gollum, the Hobbits realized that the tunnel was blocked by her webs. She cornered them, but Frodo used the Phial of Galadriel's light to drive her off.