Check Bank Tabs
If you are sure you haven't accidentally dropped your Excalibur, it is most likely just hiding in one of your bank tabs where you may have deposited it. Check all your tabs, including the ones not related to weapons. Hover the mouse over all your swords to double-check that none of them is the Excalibur.
Making Gold
To recover your lost Excalibur, you have to pay 500 gold coins. You can earn coins by selling items and materials you receive from skills such as woodcutting or mining. You can also kill monsters, retrieve the items they drop and sell them to other players on the Grand Exchange to earn this money.
Lumbridge to Falador
Players must travel to Taverly to find the non-player character (NPC) known as the Lady of the Lake. Only she can restore a lost Excalibur. Taverly is located to the Northwest of Falador. To get there from Lumbridge, head north up the path from the Lumbridge castle; you will pass goblin huts on the left, cows on the right and finally chickens on the left. Cross the fence and continue north into Varrock. Exit through Varrock's west exit. Follow the path to Falador.
Taverly
Head northwest of Falador up the path. Enter the gates to Taverly. Head to the middle of the lake at the southern part of Taverly. Talk to the Lady of the Lake and tell her that you have lost your Excalibur. Agree to pay 500 gold coins and she will hand you a new one.