Why could not he move when he woke up?

Because his arms, legs and hair were tied to the ground.

be tied to 被栓在…上 句中的were tied to…是一般过去时的被动语态。 The horse was tied to the big tree. 那匹马被拴在大树上。 The young trees should be tied to the sticks. 小树苗应该绑在棍子上。