When he was about to knight Tristian, Excalibur suddenly lost it's power, so he traveled to the Lady of the Lake in order for it to be restored. Arthur continued to use Excalibur for the remainder of his life until he was struck down in battle by the Druid King when he was trying to get the Holy Grail from his crotch so he could defeat his evil son Mordred.
Lancelot successfully killed Mordred and under the request of the dying Arthur, went to the Lady of the Lake so she could throw Excalibur into the water from whence it came. He arrived, only to find that she had gone to a magic people's convention. Lancelot then realized that with Excalibur, he could become king, but fell into the lake and drowned.
Normally, Excalibur was a large two-handed sword. However, when it lost it's power, it turned into a small golden dagger.