By the time Gandhara had been absorbed into the empire of Mahmud of Ghazni, around 1000 AD, Buddhist buildings were already in ruins and Gandhara art had been forgotten. In the 19th century, British soldiers and administrators started taking interest in the ancient history of the Indian Subcontinent. In the 1830s very old coins were discovered and decipherment of some Chinese records provided locations of Buddhists shrines. Along with the discovery of coins, these records provided necessary clues to piece together the history of Gandhara. In 1848 Gandhara sculptures have been discovered north of Peshawar. From then on a large number of Buddhist statues have been discovered in the Peshawar valley.
Se all details at http://en.wikipedia.org/wiki/Gandhara
Headless Standing Buddha, II Century AD, Museo Arte orientale, Torino